For a complete chronology of the events of the Horus Heresy please see Horus Heresy Chronology.

"It was treachery at first. To turn against brothers, to kill for personal advancement and power. But we have seen them, how their minds and bodies have been corrupted. Their very belief systems have been warped. This is no longer Horus's treachery. It is his heresy."

— Attributed to Roboute Guilliman, Lord of Ultramar and Primarch of the Ultramarines Legion





The Horus Heresy was a galaxy-spanning civil war that consumed the worlds of Mankind for 9 Terran years. Its outbreak marked the end of the Emperor of Mankind's Great Crusade to reunite the scattered colony worlds of humanity under a single government and the beginning of the current Age of the Imperium. The Horus Heresy is in many ways the founding event of the Imperium of Man as it now exists.

The civil war began following the corruption by the Ruinous Powers of Chaos of the Imperial Warmaster Horus Lupercal, the primarch of the Sons of Horus Legion and the most beloved genetic son of the Emperor. The Chaos Gods fed Horus' innate ambition and sense of betrayal by the Emperor until he turned upon his father and sought to claim the Imperium of Man for his own.

Horus convinced half of the other primarchs and Space Marine Legions, as well as large swathes of the Imperial Army and the forces of the ancient Mechanicum, to turn Traitor alongside him and unleash a cataclysmic attempt to conquer the galaxy in his name.

Over 9 Terran years the war raged, 7 of them consumed by Horus' drive on the Throneworld, until coming to a final cataclysm on Terra itself where Horus was slain and the Emperor mortally wounded, His dreams of creating a brighter future for Mankind forever broken.

The conflict was fought across the Milky Way Galaxy early in the first century of the 31st Millennium and resulted in more than 2.3 trillion dead, 4.6 trillion if one includes the planetary populations purged by the Imperium after the Heresy due to the taint of Chaos corruption.

The Heresy concluded with the death of the traitorous Warmaster Horus, the internment of the Emperor in the Golden Throne and the exile into the Eye of Terror of the Heretic Astartes Traitor Legions.

The Heresy was directly responsible for the birth of the present-day structure of the Space Marine Chapters following the Second Founding and the Reformation of the Imperium by the Ultramarines' Primarch Roboute Guilliman.

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Unity

"And in that time the Great Beasts shall walk the earth,

beneath their tread shall cities become as dust

by their burning breath shall civilisations be as ash,

So the End of Days is begun."

—The Apocrypha Terra

During the turbulent era known as the Age of Strife, the Sol System and the nearby star systems that had been colonised by humanity during the Dark Age of Technology were effectively cut off from interstellar travel or communication with each other. This was due to the massive Warp Storms that swept the galaxy as the Immaterium was roiled by the millennia-long gestation of the Chaos God Slaanesh and the turbulence that marked the decay of the Aeldari Empire before the Fall.

During this dark time, Terra sometimes held sway over the Sol System, while at other times the rulers of Mars or Luna were dominant. The different worlds found themselves constantly at war. During this 5,000 standard-year-long period of anarchy, fear and violence, Old Earth's once unified planetary government had completely broken down and been divided into dozens of warring states of so-called techno-barbarians.

Continuous warfare raged across the surface of Terra for 2,500 years, beginning in the late 27th Millennium. Little remained of the once sophisticated civilisation of Old Earth's glorious past as the centre of a growing human interstellar commonwealth marked by advanced science, high culture and wondrous technologies. Techno-barbarian warlords and their warrior hordes continuously fought over the planet, which had become little more than a massive battleground for their wars of attrition. They made use of chemical, biological and even thermonuclear weapons of mass destruction, and slowly transformed the cradle of Mankind into a battered, post-apocalyptic wasteland across most of its scarred surface.

This was a dark period for the people of Old Earth, when they were dominated by brutal rulers and despotic tyrants. It was against this backdrop of oppression, violence and casual brutality that the Emperor of Mankind first revealed Himself openly to the people of Terra. In secret, he had been planning for this moment in history for millennia, ever since the Age of Strife had fractured what remained of the ancient human interstellar civilisation.

With his massive army of genetically-enhanced warriors who comprised the first units of the Imperial Army and would serve as the prototypes for the later development of the Space Marines, the Emperor began His conquest of Terra. His intent was to reunite the warring nations of the world into a unified planetary government and then use Terra as the springboard from which to begin His reconquest of the galaxy under the aegis of an Imperium of Man dedicated to the Imperial Truth of progress and reason.

Brutal rulers such as the warlord Kalagann of Ursh, Cardinal Tang of the Yndonesic Bloc, and the most infamous of all, the half-mad, half-genius Nathaniel Dume, the tyrant of the Pan-Pacific Empire, would all fall by the wayside. Once unleashed, the geentically-enhanced armies of the Emperor swept all before them like wheat before the harvester's scythe until all the techno-barbarian warlords had either been conquered outright or had agreed through diplomacy to become subservient to the Emperor's will.

During this time the Emperor created a number of military organisations, such as the Imperial Army, which would become the nucleus of the armed forces that would later support His Space Marine Legions and His reconquest of the human-settled galaxy. Amongst the early Imperial Army units that saw action during the Unification Wars were such venerable regiments as the Geno Five-Two Chiliad, which would become one of the oldest and most respected regiments within the Imperial Army. Like many of these early regiments, the geno-soldiers were created through the use of some of the Emperor's sophisticated genetic engineering techniques, developed in His laboratories beneath the Himalazian (Himalayan) Mountains and by the advanced geneticists who called Luna home.

The most elite of this first generation of genetically-engineered supersoldiers were known as the Thunder Warriors, men who were physically stronger and more formidable in combat than even the later Space Marines, though they were engineered to be vicious killers and lacked many of the more noble aspects of the Astartes as well as their tremendously long lifespans. The Thunder Warriors were engineered to be the means to an end and were never intended to be integrated into the Emperor's new realm after Unity had been achieved.

Through genetic engineering and selection the standard geno-soldiers displayed many of the characteristics of the perfect human warrior -- they were physically more resilient, stronger and capable of taking more damage than any of their unaltered techno-barbarian foes, including the ability to face down rogue psykers and sorcerers. These early genetically-engineered Imperial Army regiments would go on to continue serving the Emperor after the conclusion of the Unification Wars.

These regiments would eventually be referred to as the Old Hundred, and would form the core of the initial military force that embarked upon the Emperor's galaxy-spanning Great Crusade in the late 30th Millennium beside his newly created Space Marine Legions, the early versions of which had taken part in the later campaigns of the Wars of Unification.

When the Unification Wars were complete in the late 30th Millennium, the Emperor forged a new unified planetary government for Terra under His leadership. He next journeyed to Mars and met with the tech-adepts of the Cult Mechanicus.

In return for the use of the Mechanicum's vast manufactoria, the use of its Titan Legions and the orbital shipyards to construct the weaponry and starships he would need for His Great Crusade to reunite Mankind under the banner of the Imperial Truth, the Emperor agreed to grant the Mechanicum complete autonomy on Mars and its other Forge Worlds as well as an exemption to the atheism required by the Imperial Truth.

This agreement, known as the Treaty of Mars (the Treaty of Olympus to the Mechanicum), marks the true foundation of the Imperium of Man in the alliance between Terra and Mars.

Council of War

Following the successful conclusion of the Unification Wars, the Emperor convened the ruling body known as the War Council to manage the execution of the Great Crusade intended to reunite the entire human-settled galaxy under a single government. The War Council effectively became the true ruling body of the Imperium during the early and middle years of the Great Crusade.

The Emperor Himself sat at the head of the council; at his left hand was Malcador the Sigillite, perhaps the Emperor's greatest ally during the Wars of Unity and a human psyker whose powers were matched only by those of the Emperor.

The rest of the Council was composed of talented administrators drawn from the great ruling aristocratic dynasties of Terra and the Segmentum Solar, and when the Emperor forged His alliance with the Mechanicum of Mars, the Fabricator-General of the Mechanicum also claimed his seat. The War Council was also attended by the Paternova of the Navigator Houses.

As the Emperor departed the homeworld of Mankind to lead the Great Crusade into the stars starting around ca. 798.M30, he left the legendary Malcador to act as the Regent of Terra and the head of the War Council in his stead.

At the same time, Malcador took the lead in creating the clades of Imperial Assassins that would eventually evolve into the Officio Assassinorum, making Malcador the first Grand Master of Assassins.

Great Crusade

When the great Warp Storms that had cut off Terra since the end of the Dark Age of Technology subsided, and the Age of Strife came to an end at the dawn of the 31st Millennium, the Emperor of Mankind deemed it time to begin his Great Crusade, a massive campaign to conquer the galaxy by which he and his armies would free all human-settled colony worlds from alien oppression or primitive ignorance and reunite the human race across the galaxy under the single banner of the new Imperium of Man. To execute this plan, the Emperor created the primarchs, his god-like, genetically-engineered superhuman offspring.

The primarchs were still in their infancy, growing to hyper-accelerated maturity in their special gestation tanks, when they were snatched away from the genetic laboratory deep beneath the Himalazian (Himalayan) Mountains on Terra where they had been created and gestated by the Emperor using His own DNA. The cause of this disappearance was the Chaos Gods, who were fearful that with the primarchs, the Emperor would be able to impose His new order across the galaxy and weaken their own firm grip on the collective unconsciousness of the minds of Humanity, which was the source of both their existence and the growth in their power within the Immaterium.

Uniting their powers under the leadership of Tzeentch, the Dark Gods opened a portal from the Warp and broke into the laboratory where the primarchs were gestating through the potent psychic wards the Emperor had erected. Unable to destroy the primarchs outright because of the protections the Emperor had laid upon them, the Chaos Gods instead chose to scatter them across the galaxy through the Warp.

The superhuman infants eventually came to rest on diverse, human-inhabited worlds. It was at this time that the Ruinous Powers first touched the souls of those primarchs who would eventually turn to the worship of Chaos, infecting them with a shard of corruption.

Certainly the fact that each of the primarchs was ultimately cast ashore on an inhabited human world was no accident; the Chaos Gods may have hoped that by having the primarchs raised among Humanity without the direct guidance of the Emperor as they grew they would display more of the human weaknesses that would make them easier to corrupt when the time came.

During the course of the Great Crusade, the Emperor encountered each of the primarchs on their scattered homeworlds in turn. To fill the gap in His military plans for the Great Crusade wrought by the primarchs' absence after they had been stolen away, the Emperor had instead created 20 Space Marine Legions, several of which, such as the [[Ist Legion]], had been raised even as the Unifications Wars still raged.

The Emperor used the primarchs' individual genetic material still in His possession to craft the genetically-enhanced transhuman warriors of each of the Legiones Astartes, creating 19 gene-seed organs that could be implanted within an adolescent Human male to transform him into a Space Marine. After the Emperor rediscovered His sons scattered across the galaxy, He deemed it fitting that each primarch should lead their genetic offspring as the master of the Legion whose Astartes bore their DNA.

However, in time this decision would prove a critical mistake, as the Space Marine gene-seed creation process made the Emperor, the primarchs and the Space Marines analogous to grandfather, fathers and sons, respectively, in their genetic inheritance and superhuman abilities.

In time, many of the Space Marines in the Legions, especially those recruited from their primarchs' homeworld rather than from Terra before the primarchs' rediscovery, would come to venerate and feel more loyalty for their primarch than the Emperor of Mankind.

Triumph of Ullanor

"You are like a son and together we have all but conquered the galaxy. Now the time has come for me to retire to Terra. My work as a soldier is done and now passes to you for I have great tasks to perform in my earthly sanctum. I name you Warmaster and from this day forth all of my armies and generals shall take orders from you as if the words cam from mine own mouth. But words of caution I have for you for your brother Primarchs are strong of will, of though and of action. Do not seek to change them, but use their particular strengths well. You have much work to do for there are still many words to liberate, many peoples to rescue. My trust is with you. Hail Horus! Hail the Warmaster!"

—The Emperor of Mankind, at the Triumph of Ullanor

The Ullanor Crusade was a vast Imperial assault on the Ork empire of the Overlord Urrlak Urruk during the late 30th Millennium. This massive undertaking would mark the culmination of the Great Crusade's expansion.

Ullanor, the capital world of this empire, and the site of the final assault, lay in the Ullanor System of the galaxy's Ullanor Sector. The Crusade included the deployment of 100,000 Space Marines, 8,000,000 Imperial Army troops, and thousands of Imperial starships and their support personnel. The Ullanor Crusade marked the high point of the Great Crusade's vast effort to reunite the scattered colony worlds of humanity. The Orks of Ullanor represented the largest concentration of Orks ever defeated by the military forces of the Imperium of Man before the Third War for Armageddon began during the late 41st Millennium.

With the great victory sealed in blood and iron, the call to a Triumph was sounded, to recognise this highpoint of the Great Crusade and to honour all the warriors of the Ullanor Crusade, mortals and Astartes alike, for their extraordinary valour and service to humanity's cause. By the Emperor’s command, Ullanor was remade as a trophy world, designated Mundus Tropaeum on all galactic maps and records of tithe. It would be a site of glory and spectacle to cement not only this single conquest over the forces ranged against Mankind, but a greater symbol of the Great Crusade itself. For two hundred Terran years, the Emperor’s mighty endeavour had moved across the face of the galaxy to bring unity and illumination to the lost daughter-worlds of Old Earth. It had pushed back the night, reforged old links between civilisations, battled alien threats -- and with regret, it had often punished those who refused to return to the Imperial fold. A change was coming, though, a change that found its fulcrum on Ullanor. None who walked upon that world knew that the echo of that Triumph would sound for decades, for centuries, for millennia. The glory of this triumphant spectacle as so many of the Imperium's scattered military forces gathered in one place for the first time in centuries was to remain in the mind of every Astartes as the high-point of the great endeavour that they were engaged upon. It would prove to be a bright memory to recall in the dark days of the Horus Heresy after Astartes had turned against Astartes and Primarch against Primarch.

To prepare the world for the Imperial Triumph, geoformer platoons from the Adeptus Mechanicus brought world engines and mobile stone-burners to cut a massive swath across the broken landscape left in the battle’s wake. Orkish dead were buried by the millions with their savage ruins, interred beneath transplanted rocks and the heads of crushed mountains. The Mechanicus eradicated every last remaining trace of the enemy and paved over them with a giant boulevard, a parade stage as wide as the footprint of some entire Imperial cities. They built a highway and allowed only one structure to stand besides the great platform -- an ornamental pavilion of black marble and heavy granite that had been built piecemeal on Terra and then shipped across the void by special envoy. Marker posts decorated with the skulls of Ork commanders paced out the length of the road, and behind them great bowls of smokeless Promethium burned brightly, endlessly lighting the highway with their blue-white fire.

When the Mechanicus had finished their work, the honoured came to pay homage to the battle won, the Great Crusade’s ideal of human unity and the Emperor who was father to all Mankind. The Imperial Army and the Titan Legions bracketed the gathering. Human troops were ranked in uncountable numbers, their host so wide they became a sea of battle armour and dress uniforms. Every common man and woman who stood on Ullanor’s soil that day had been selected for their valour and conduct, and until the day they died each would have the singular honour of wearing the onyx-and-gold Ullanor Triumph Bar upon their uniforms. The award was forged from Bolter shells recovered from the field and melted down. Ranged around them, the great war machines of the Collegia Titanica towered towards a sky cut to ribbons by the contrails of a thousand aerospace fighters; and above those, high over the thin white cirrus clouds of Ullanor’s day, Imperial warships moved as slow as they dared through the upper atmosphere, washes of interface heat rolling off their Void Shields as they showed their flanks in a gesture of renewed fealty.

A full fourteen of the Space Marine Legions stood represented at the Ullanor Triumph, and with them came nine beings of superhuman power and majesty. Nine gods and angels made flesh, the Primarchs of the greatest armies ever created by human hands. Mortarion, the reaper of men and master of the Death Guard, cowled and lethal in aspect, matched by the warrior-guardians of his Deathshroud honour guard. The Phoenician, Fulgrim, resplendent in his finery and handsome in aspect, lit by the reflection of gold and platinum. Magnus the Red, the Crimson King, the lord of the unknown, his soul as much a mystery to the common world as the workings of the Warp and the ghosts within it. Lorgar Aurelian, the quiet and brooding zealot who burned with such intensity and buried it all deep in his heart, saying little and standing watchful. His polar opposite was Angron, the gladiator-lord and son of grief, never able to settle or moderate his seething, endless fury, always on the verge of outburst and violence. Rogal Dorn, the stalwart man of stone, the Imperial Fist with his unswerving manner and unbreakable focus, the one who would always obey, would always be ready for duty. Jaghatai Khan, his fur-trimmed robes and ornate armour detailed with a thousand narratives of the White Scars Legion, his every step across the land a challenge to the galaxy. Then Sanguinius of the Blood Angels, flanked by the gold-armoured honour detail of the Sanguinary Guard, his mighty wings folded back across his battle-plate, his face turned to the sky to welcome the impossible, majestic sight before him. Then, finally, came Horus of the Luna Wolves, the Hero of Ullanor, liberator and first among equals. Horus, who was to be given the new honour of an Imperial title above and beyond any that had been bestowed before; a title, it could be said, that would forever carry the echo of his name. After declaring Horus as Imperial Warmaster and the new supreme commander of the Great Crusade, the Emperor to the shock of those assembled announced his own intention to return to Terra to pursue a secret project intended to benefit all Mankind.

Seeds of Heresy Sown

After more than 200 standard years of hard conflict by the early 31st Millennium, over two million human-settled worlds across the Milky Way Galaxy had been reclaimed by the Emperor in the name of the Imperium of Man. Beside him had stood the Primarch Horus, who had fought alongside the Emperor for the early part of the Great Crusade as his only rediscovered son and primary military commander, glorying in the singular attentions of his father. The long wars had forged a strong bond between them, and they were truly father and son. But now the Emperor had to consolidate his newborn Imperium, and undertake the next phase of his Grand Plan. He intended to create a Webway through the Immaterium much like that used by the Eldar that would be open to humanity by wielding the psychic augmentation technology of the artefact from the Dark Age of Technology known as the Golden Throne. This work required his continued presence on Terra, and so after Horus' magnificent victory in the Ullanor Crusade, against the largest horde of Orks ever encountered in the galaxy at that time, the Emperor departed and left Horus in charge of the Great Crusade with the exclusive title of "Warmaster." As Warmaster, Horus was the commanding general of all of the Imperium of Man's military forces, charged with leading the other Primarchs and their Legions through the remainder of the Great Crusade.

At this announcement there was much shock and outrage. Many of the other Primarchs did not understand why the Emperor was leaving them to fight the enemies of Mankind alone and, worse still, why Horus should be raised as the first amongst equals. Rogal Dorn, Sanguinius and Fulgrim were pleased for their brother, the new Warmaster, while others -- such as Angron, Roboute Guilliman, Lion El'Jonson and Perturabo -- all reacted with varying degrees of disapproval. The situation only grew worse when the Emperor announced that he would be creating a civilian administrative bureaucracy known as the Council of Terra that was comprised of Imperial bureaucrats and nobles to carry out the day-to-day governmental affairs of the Imperium, replacing the direct rule of the Emperor while he was engaged in his secret Imperial Webway Project. The Council of Terra would implement and administer the new galaxy-wide tax of resources and manpower called the Imperial Tithe and other matters of day-to-day law in the Imperium of Man while the Emperor focused on bringing his Webway Project to fruition using the technology of the Golden Throne. The Primarchs would be relegated to a primarily military role as the Imperium's most preeminent commanders. Many of the Primarchs, including Horus, were deeply disturbed that their father would make them subject to normal men and women who had never shed blood in the establishment or expansion of the Imperium and that he would remove them from the political positions of rule to which they believed they were entitled. The Emperor had sought to create a civilian bureaucracy for the Imperium precisely because he wanted regular human beings to learn to govern themselves once more and not become beholden to a permanent, genetically-enhanced ruling class. The Chaos Gods would ultimately use the Primarchs' resentment as one of the all-too-human weaknesses they could exploit to corrupt half their number.

Lords of the Imperium

After the decisive victory during the Ullanor Crusade, when Mankind's re-ascension to predominance in the galaxy was no longer in doubt, the Emperor bestowed upon the Primarch Horus Lupercal the title of Imperial Warmaster and ceded to him control of all the Imperium's military forces in the Emperor's stead. The other Primarchs were then instructed to follow Horus and obey him and complete the Great Crusade under his direction. There was, it is said, some disquiet among the Primarchs that the Emperor had decided to no longer fight alongside them, but the Emperor was as adamant as He was close-mouthed as to what He would do on his return to Terra. The Emperor then departed for the homeworld of Mankind and the dungeons deep beneath his great Imperial Palace to begin His great work on the creation of a human extension into the Eldar Webway under a veil of secrecy previously unknown in the Imperium. He drew to him certain advisors and retired to the private vaults deep beneath his city-fortress within the Imperial Palace.

Upon his return to Terra, the Emperor called to his side Malcador and the Fabricator-General of the Mechanicum. He issued them with new commands. No longer were they to support the military campaigns, as these were now safely in the hands of his sons the Primarchs and the newly appointed Warmaster Horus. The Emperor needed time and all of his focus to be directed at his next great project, which would tie the newborn Imperium together and unite it as no other human polity had ever been united. To this end, the Emperor convened the first Council of Terra. Unlike the War Council, of which Horus was now the leader, the Council of Terra would attend to matters of state and the establishment and maintenance of Imperial Law across the myriad worlds of the Imperium. In particular, the Council of Terra was to administrate the establishment of the Imperial Tithe of troops and resources from all the worlds of the Imperium that were required to support the Great Crusade. In effect, to the new Council of Terra would fall the entirety of the civil government of the Imperium. Malcador, the Emperor's most trusted advisor, was named as the First Lord of the Council and would lead it in the Emperor's absence. The Fabricator-General of the Mechanicum of Mars, Kelbor-Hal, Captain-General Constantin Valdor of the Legio Custodes and the Masters of the Adeptus Astronomica, the Adeptus Astra Telepathica and the Administratum of the Imperium were also appointed to the Council.

Having established the new governing body of the Imperium to carry out the day-to-day work of ruling tens of thousands of worlds and trillions of human beings, the Emperor took refuge in His vast laboratories and workshops beneath the Imperial Palace. He began work in earnest on his new project. While the Emperor was locked away in His subterranean factories, trouble was brewing. The formation of the Council proved to be a contentious decision with the distant Primarchs, who were appalled when news of the formation of the Council of Terra finally reached them on the frontiers of the Great Crusade. Some of the Primarchs took great exception to being ruled by those they deemed less worthy of such an honour than themselves. The less stable Primarchs felt that this was a betrayal of all they had fought and won in the Emperor's name and that their victories now counted for nothing. The Primarchs, and many of their Astartes, felt that it was they who had suffered and sacrificed the most to build the Imperium and thus it was they who should have the greatest say in how it was ruled, not a council composed of effete Terran nobles and faceless bureaucrats. This was one of many growing resentments that allowed the Ruinous Powers to infect and corrupt several of the Primarchs.

The creation of the Council of Terra, seen in this light, lent new weight to Horus' argument to several of his brothers that the Imperium had been betrayed by their father. He argued that the Emperor had proved more than willing to turn His back on his sons and generals and give power instead to petty mortal administrators and the sycophantic Tech-adepts of Mars who lacked the Primarchs' brilliance and superhuman abilities. As Horus prepared his rebellion against the Emperor, he convinced himself that petty functionaries and administrators had supplanted the Primarchs and the Astartes within the Imperium they had won. Once the Imperium had been wholly geared for the war and conquest that was its life's blood since its inception, but now Horus believed that it had become burdened with parasitic exectors, scribes and scriveners who demanded to know the cost of everything. Bureaucracy was taking over -- red tape, administrators and clerks were replacing the heroes of the age. Horus argued to his more receptive brothers that unless the Imperium changed its ways and direction, its greatness as an empire would soon be a footnote in history books. Horus feared that everything he and his brother Primarchs had achieved would be a distant memory of former glory, lost in the mists of time like the civilisations of ancient Terra. It was this hubris and arrogance that led to the Warmaster's inevitable fall to Chaos and the resultant civil war that would consume the entire galaxy, ushering in a new Age of Darkness that would last for millennia.

Yet Horus and his more resentful brothers had completely misunderstood the Emperor's intent in creating the Council of Terra. The Council was to become the body of civilian government that would administrate the myriad bureaucratic tasks needed for the survival of the newly formed human interstellar empire. The Emperor was determined that in His Imperium power would reside with those men and women who were governed by its apparatus and not with an artificial military elite composed of genetically-engineered beings who were so powerful that they already possessed only a very tenuous grip upon their own humanity. The Primarchs and their Space Marines had been created to give life to the Emperor's dream of a united human Imperium stretching across the galaxy and to defend it from humanity's myriad foes. They were not to rule it as a hereditary caste of immortal warriors imposing their whims upon those they deemed mere "mortals" by brute force.

Corruption of the Space Marine Legions

Long before the tragic events that would unfold on Istvaan III and initiate the conflict of the Horus Heresy, the Primarch Lorgar of the Word Bearers Legion had already committed himself and his Astartes to the service of the Ruinous Powers. Lorgar was a puritanical religious zaelot who was said to have experienced visions that foresaw the coming of the Emperor, who he believed to be a living God. This belief resulted in a series of bitter religious wars as Lorgar fought to impose his new religious doctrine worshipping the Emperor on his homeworld of Colchis. When the Emperor finally arrived to reclaim his lost son, the entire world was already enthralled to Lorgar and his Cult of the Emperor. The people of Colchis united behind their new living God. The elaborate celebrations and displays of piety lasted for months, although it was said that the Emperor did not approve of this, wishing to rejoin the Great Crusade as soon as possible and being greatly dismissive of organised religion in general. The Emperor had not begun the Great Crusade to reshackle humanity within the chains of superstition and ignorance but to spread the light of reason and science. At the conclusion of the celebrations, Lorgar was made commander of the XVII Space Marine Legion, the Imperial Heralds, who were renamed the Word Bearers after they embraced their Primarch and his religious beliefs. Kor Phaeron, Lorgar's adoptive father and religious advisor, survived the augmentation process to join the Word Bearers as a rare adult Space Marine, though he would never be a true Astartes. Kor Phaeron became Lorgar's chief adviser, lieutenant and the commander of the Word Bearers' elite 1st Company as the XVII Legion's First Captain.

Lorgar led his Legion throughout the Great Crusade, as the Word Bearers sought to eliminate all blasphemy and heresy within the new Imperium of Man. Ancient texts and icons of other faiths were burned. The construction of vast monuments and cathedrals venerating the Emperor as the God of Mankind were supervised by Lorgar and the Word Bearers on many of the worlds they brought into Imperial Compliance. The greatest Chaplains of the Word Bearers produced enormous works on the divinity and righteousness of the Emperor, and gave grand speeches and sermons to the masses of conquered worlds. The progress of the Word Bearers was slow in bringing new worlds into Imperial Compliance, but the Imperial domination of those who were defeated by the XVII Legion was always complete, as religion proved to be a potent tool in securing Imperial loyalty. At some point during this period, Lorgar penned the work known as the Lectitio Divinitatus, which laid out the case that the Emperor of Mankind was a divine being and was worthy of worship as the rightful God of humanity. This book would later, ironically considering the identity of its author, become instrumental in the founding of the Imperial Cult and the Ecclesiarchy.

Castigation of Khur

At this time, some 40 standard years before the start of the Horus Heresy, the absolute loyalty of Lorgar and the Word Bearers Legion to the Emperor and his Imperium was unquestioned. Their Compliant worlds regularly delivered tithes in the Emperor’s name, and the orders of Terra were accepted without question throughout the worlds liberated by the Word Bearers. Lorgar and his Legion had successfully prosecuted the Emperor's Great Crusade for almost a full Terran century, and in that time the Emperor had never once admonished His zealous son or the Word Bearers Legion for their fervent worship of Him even though such doctrine clashed with the Emperor's Imperial Truth.

But the Emperor, for all His love for His son, was deeply disturbed by Lorgar's unwillingness to embrace reason or alter his practices despite decades of exposure to the advanced science and technology of the Imperium. He had initially tolerated the beliefs of his deeply religious son, but as the Great Crusade reached its height, the Emperor found Himself increasingly frustrated with the slow pace with which Lorgar conquered and then brought worlds into Compliance for the Imperium. The Emperor finally ordered the Word Bearers to cease their religious activities, as their mission was to reunify the galaxy under the banner of the secular Imperial Truth, not preach the word of the Emperor's personal divinity. The Emperor had long opposed the spread of organised religion and was determined to use the creation of the new Imperium of Man to enshrine reason and science, not religion, as the true guiding light of a new interstellar human civilisation. The Emperor was particularly troubled by any notion that He should be worshipped as a God and the actions of the Word Bearers Legion in slaughtering those who refused to accept the Emperor's divinity stank of the religious excesses that had so often poisoned human history.

The Emperor ordered a task force composed of the Ultramarines Legion and lead by their Primarch Roboute Guilliman and accompanied by a force of his elite personal bodyguards, the Legio Custodes and the Imperial Regent, Malcador the Sigillite, to raze the capital city of the planet Khur, a world dear to the Word Bearers, who considered its capital, Monarchia, the "perfect city" because of the intense religious devotion of its citizens and the sheer number of cathedrals and monuments dedicated to the worship of the Emperor as a God. Following the city's destruction by the Ultramarines, the entire Word Bearers Legion, 100,000 Space Marines strong, were ordered to assemble on the planet's surface, within sight of the smoldering ruins of Monarchia, where its Astartes were humiliated and rebuked by the Emperor Himself, who psychically forced everyone, including Lorgar, to kneel before Him, and explained to them that they had failed both Him and humanity. Lorgar was stunned by his father's reproach and refusal to accept his worship, and fell into a deep melancholy.

Feeling betrayed by the Emperor, Lorgar refused audience to all but Kor Phaeron, the Word Bearers' First Captain and Cardinal of its faith. Kor Phaeron was Lorgar's adoptive father and had raised him from infancy on Colchis as a member of the Colchisiam religious order called the Covenant. Kor Phaeron had served as Lorgar's chief lieutenant and advisor since the time when he ruled as the theocrat of Colchis. Lorgar also called the the Legion's First Chaplain Erebus to his side, who had long been another trusted advisor. Kor Phaeron and Erebus sympathised with Lorgar's unrequited religious longings, and felt that the Word Bearers Legion should serve gods truly worthy of their worship. Kor Phaeron and Erebus explained that they knew of such gods, the divine beings once worshipped by the Old Faith of Colchis. Thus, it was in this way that Lorgar first learned of the existence of the Chaos Gods, who not only accepted the zealous worship he offered, but demanded it. Thus the seeds of the Horus Heresy were first sown amongst the Word Bearers. Intrigued, Lorgar demanded that the Legion find these gods, and Kor Phaeron and Erebus, both of whom had been secret devotees of Chaos for decades, proposed a pilgrimage.

Pilgrimage of Lorgar

Prompted by First Captain Kor Phaeron and the XVII Legion's First Chaplain Erebus, both secret devotees of the Chaos Gods through Colchis' Old Faith, Lorgar journeyed with his Word Bearers Legion's Chapter of the Serrated Sun to what was then the fringes of known Imperial space as part of the 1301st Expeditionary Fleet of the Great Crusade. At this time, Lorgar had not yet fallen to the corruption of Chaos, though he had turned against the Emperor of Mankind as a deity no longer worthy of his worship. Lorgar believed that the Emperor was wrong to condemn Mankind's natural instinct to seek out the divine as an unworthy superstition and he intended to discover if there were truly deities worthy of humanity's respect. To this end, though Lorgar no longer had any love or loyalty for the Emperor, he and his XVII Legion rejoined the Great Crusade but did so only so their efforts could serve as a front for their pursuit of the Pilgrimage of Lorgar.

The Word Bearers were also accompanied on this Pilgrimage by 5 members of the Legio Custodes who had been set by the Emperor to watch over everything the Word Bearers did to prevent them from falling back into error once more. The Word Bearers' pursuit of any scrap of information that could be found on the Primordial Truth or the nature of the place where Gods and mortals could mingle ultimately led the 1301st Expeditionary Fleet to the Cadia System near the largest permanent Warp Storm in the galaxy, later known to the Imperium as the Eye of Terror. The Expeditionary Fleet's Master of Astropaths advised Lorgar that unusual "voices" in the Warp were heard in the vicinity of the great Warp rift, voices that spoke directly to the Primarch as well, which were the voices of the Chaos entities within the Immaterium. It would be in the Cadia System that Lorgar would learn that his suspicions had been correct and that the shape of all of the religions across the galaxy that possessed so many similarities to the Colchisian Old Faith were not artefacts of Mankind's collective unconsciousness, but expressions of worship in the universal truth that was Chaos.

Primordial Truth

The so-called "Primordial Truth" of the existence of Chaos changed Lorgar and the Word Bearers forever as they were exposed to the Ruinous Powers and slowly corrupted, the first of the Space Marine Legions to worship the Chaos Gods and become Traitors to the Emperor in their hearts. Lorgar and the Word Bearers spent the remaining years of the Great Crusade attempting to enlighten humanity about the true spiritual nature of Creation, ultimately resorting to manipulation and deception to sway nine of the Primarchs to the cause of Chaos as their Gods demanded, the most notable being the Warmaster Horus. When it became clear that Mankind could not be enlightened by Chaos without first being forcibly weaned at a great price in blood from the Emperor's false Imperial Truth, Lorgar would go on to willingly help orchestrate the terrible Battle of Istvaan III and the Drop Site Massacre at Istvaan V as well as the larger Horus Heresy itself. When Horus openly declared his rebellion against the Emperor, the Word Bearers were one of the first Legions to support him and his cause. The worlds they had conquered since their conversion to Chaos also joined the side of the Traitors, having been secretly corrupted to the worship of the Ruinous Powers in the final days of the Great Crusade.

Fall of Horus

During the Great Crusade, it became apparent that the Primarchs were far from the perfect specimens of humanity they were intended to be. Although each Primarch was physically and mentally god-like compared to a baseline human being, they harboured the flaws of vanity, egotism, hunger for power, jealousy, arrogance, insecurity and all the other sins of the human character.

As the Warmaster, Horus took over command of the Great Crusade, and accepted his new duties with earnest dedication. However, there was much dissension in the ranks of the Primarchs and other parties in the Imperium over the Emperor's decision to withdraw from the campaign and return to Terra as well as to reorganise the administration of the Imperium. Only a handful of the Primarchs, among them a scheming Lorgar, remained steadfast beside the Warmaster during this period of conflict. Horus also disagreed with many of the decrees passed by the newly established Council of Terra, intended to shift the burden of taxation and administration onto the newly-conquered ("Imperial Compliant") worlds. Even worse, Horus came to believe in his heart that he was failing his father, and was deeply wounded that the Emperor had revealed to none of the Primarchs, not even his most favoured son, why he had secluded himself upon Terra and the truth behind his secret Webway Project. These seeds of bitterness, resentment and frustration grew, and would soon bear deadly fruit.

It was on the moon of the world of Davin that Horus' fate was sealed. This was the second time his Legion had been posted to this world; after the previous visit sixty years earlier the Luna Wolves had adopted the native Davinite institution of warrior lodges. Though these lodges had begun as simple fraternities of warriors, their secretive nature handed Lorgar, the Primarch of the Word Bearers Legion and his First Chaplain Erebus, the tool they needed to manipulate Horus towards the service of the Chaos Gods.

Lorgar and his Word Bearers originally came from Colchis, a world defined by religious fanaticism, and had long worshiped the Emperor as a god. The Word Bearers had sought to spread their Cult of the Emperor to every world they added to the Imperium. But the Emperor deeply disliked and mistrusted organized religion (ironic, since he had often been the focus for much of it in his various guises across history), blaming it for much of the darkness that had plagued humanity's history. The Emperor openly and publicly refuted his alleged divinity and banned religious worship in his empire, and demanded that his subjects accept the "Imperial Truth"-- that science, reason and logic alone presented the tools required to create a better human future. Lorgar did not suffer the Emperor's reprimand or views on religion well. Angered and wounded that the Emperor would not accept his devotion and worship, Lorgar turned instead to the Ruinous Powers of the Warp -- who were all too willing to accept the devotion of one of humanity's Primarchs. Before long, the Word Bearers Legion had been almost entirely corrupted by the Chaos Gods, and Lorgar and Erebus were tasked by the Ruinous Powers with corrupting all of their fellow Space Marines -- starting with the greatest of them all, the Warmaster Horus.

During a battle against Chaos-spawned undead on Davin's moon, whose Planetary Governor, Eugen Temba, had been corrupted by the forces of the Chaos God Nurgle, Horus was poisoned by a xenos blade dedicated to Nurgle known as a Kinebrach Anathame that had been stolen from the human civilisation of the Interex by Erebus after Horus and the Luna Wolves of the 63rd Expeditionary Fleet had made a disastrous first contact with them. Erebus then gifted the weapon to the Chaos-corrupted form of the Imperial Army commander Eugen Temba who the Warmaster had left behind to govern Davin sixty years before. Temba had turned to the worship of Nurgle in the interim, being transformed into a bloated mutant and killing off most of his Imperial Army garrison in the process, transforming them into undead Plague Zombies that the Luna Wolves were forced to mow down in waves. Horus personally faced off with the mutant that had been Temba aboard the grounded ruins of his Imperial Cruiser. In the course of that battle, the potent living metal of the Chaos blade wielded by the plague-infused monstrosity left Horus with a bleeding, toxic wound in his shoulder that his Legion's Apothecaries could not heal despite all the advanced technology available to them. Seeing his chance to further the designs of Chaos, Erebus next persuaded the Luna Wolves' warrior lodge to allow a group of Davinite shamans -- Chaos Cultists all -- located on the surface of Davin at the Temple of the Serpent Lodge to heal him. The Luna Wolves, besides themselves with grief and the fear that their beloved Primarch would die, agreed to the suggestion, despite its direct violation of the creeds of the Imperial Truth.

During the dark rituals that followed within the temple, Horus' spirit was transferred from his body into the Immaterium. There, he bore witness to a nightmare vision of the future. He saw the Imperium of Man as a repressive, violent theocracy, where the Emperor and several of his Primarchs (but not Horus) were worshiped as Gods by the masses. While this vision of the Imperial future granted by the Chaos Gods was a true one, it was ironically an outcome largely created by the Warmaster's own actions. The Dark Gods portrayed themselves as victims of the Emperor's psychic might, and claimed that they had no real interest in the happenings of the material world. Magnus the Red, the sorcerous Primarch of the Thousand Sons Legion, had also travelled into the Warp via sorcery to try and stop Horus from turning to Chaos. Magnus explained that the Warmaster's vision was only one of many possible futures, but one that Horus alone could prevent. Horus, already jealous and resentful of the Emperor, proved all too receptive to the Ruinous Powers' false vision. The Chaos Gods' pact with Horus was simple: "Give us the Emperor and we will give you the galaxy." Driven by his jealousy, desire for power and anger at what he saw as his father's abandonment of him, Horus accepted the Ruinous Powers' offer. They healed his grievous wound and filled him with the powers of the Warp. Renouncing his oath to the Emperor, Horus led his Legion, renamed the Sons of Horus, into worship of the myriad Chaos Gods in the form of Chaos Undivided. He then sought to turn many of his fellow Primarchs to the service of Chaos, and succeeded with Angron of the World Eaters, Fulgrim of the Emperor's Children and Mortarion of the Death Guard, who were the first of many to follow, along with many regiments of the Imperial Army and several Titan Legions of the Adeptus Mechanicus.

Magnus the Red, the Primarch of the Thousand Sons Legion, foresaw Horus' actions through his Legion's own use of psychic sorcery, which had been forbidden to the Space Marines and the Primarchs by the dictates of the Council of Nikaea. Magnus then attempted to forewarn the Emperor of the impending betrayal of his favourite son. However, knowing that he would have to find a means of quickly warning the Emperor, Magnus used sorcery to send his message to the Emperor. The message penetrated the potent psychic defences of the Imperial Palace on Terra, shattering all the psychic wards the Emperor had placed on the Palace -- including those within His secret project in the Imperial Palace's dungeons, where He was proceeding with the creation of the human extension into the Webway. Refusing to believe that Horus, His most beloved and trusted son, would actually betray Him, the Emperor instead mistakenly perceived the traitor to the Imperium to be Magnus and his Thousand Sons, who had long suffered from a near-debilitating run of mutations because of the instability of Magnus' own genome as well as being practicioners of sorcery that brought them into constant contact with the dangerous entities of the Empyrean. The Emperor ordered the Primarch Leman Russ, Magnus' greatest rival, to mobilise his Space Wolves Legion and the witchhunters known as the Sisters of Silence and take Magnus into custody to be returned to Terra to stand trial for violating the Council of Nikaea's prohibitions against the use of sorcery within the Imperium. While en route to the Thousand Sons Legion's homeworld of Prospero, Horus convinced Russ, who had always been repelled by Magnus' reliance on psychic powers, to launch a full assault on Prospero instead even though Magnus had been entirely willing to face the Emperor's judgment once he realised he was being manipulated by the entities that called the Immaterium home.

Istvaan Massacres

Note: Published materials are inconsistent on their spelling of "Isstvan"; the more recently published material uses "Isstvan", while other (generally older) materials use "Istvaan". No explanation for this difference has been provided. For the sake of expediency, this article will use the spelling Istvaan, with no claims made to the accuracy of the spelling.

Istvaan III Atrocity

"Horus is the rightful Master of Mankind! He is the one who has led us to triumphs undreamed of. He is the one who has conquered ten thousand worlds. He will lead us in conquest of ten thousand more! Cast down the false Emperor! Hail the Warmaster!"

— Attr. Anonymous Traitor Legionary

Unknown to the Emperor, the Word Bearers Legion had been devoted to Chaos Undivided for some time before this event. The Imperial Planetary Governor of Istvaan III, Vardus Praal, had been corrupted by the Chaos God Slaanesh whose cultists had long been active on the world. Praal had declared his independence from the Imperium, and practiced forbidden sorcery, so the Council of Terra charged Horus with the retaking of that world, primarily its capital, the Choral City. This order merely furthered Horus' plans to overthrow the Emperor. Although the four Legions under his direct command -- the Sons of Horus, the World Eaters, the Death Guard and the Emperor's Children -- had already turned Traitor and now pledged themselves to Chaos, there were still some Loyalist elements within each of these Legions that approximated one-third of each force; many of these warriors were Terran-born Space Marines who had been directly recruited into the Astartes Legions by the Emperor himself before being reunited with their Primarchs during the Great Crusade. Horus, under the guise of putting down the religious rebellion against Imperial Compliance on the world of Istvaan III, amassed his troops in the Istvaan System.

Horus had a plan by which he would destroy all the remaining Loyalist elements of the Legions under his command, a plan that would ultimately unfold into the nightmare of what Imperial scholars would later name the Istvaan III Atrocity. After a lengthy bombardment of Istvaan III, Horus dispatched all of the known Loyalist Astartes down to the planet, under the pretense of bringing it back into the Imperium. At the moment of victory and the capture of the Choral City, the planetary capital of Istvaan III, these Astartes were betrayed when a cascade of terrible virus-bombs fell onto the world, launched by the Warmaster's orbiting fleet. Captain Saul Tarvitz of the Emperor's Children, however, was aboard his Legion's flagship Andronius and discovered the plot to wipe out the Loyalist Astartes of the Traitor Legions. He was able, with help from Battle-Captain Nathaniel Garro of the Death Guard who was in command of the Death Guard frigate Eisenstein, to reach the surface of Istvaan III despite pursuit and warn the Loyalist Space Marines he could find of all four Legions of their impending doom. Those that heard or passed on Tarvitz's warning took shelter before the virus-bombs struck. The civilian population of Istvaan III received no such protection: eight billion people died almost at once as the lethal flesh-dissolving virus called the Life-Eater carried by the bombs infected every living thing on the planet. The psychic shock of so many deaths at one time shrieked through the Warp, briefly obscuring even the Astronomican. The Primarch of the World Eaters, Angron, realising that the virus-bombs had not been fully effective at eliminating all the Loyalists, flew into a rage and hurled himself at the planet with 50 companies of Traitor Marines. Discarding tactics and strategy, the World Eaters Legion's Traitors worked themselves into a frenzy of mindless butchery. Horus was furious with Angron for delaying his plans, but the Warmaster sought to turn the delay into a victory and was obliged to reinforce Angron with troops from the Sons of Horus, the Death Guard, and the Emperor's Children. Fortunately, a contingent of Loyalists led by Battle-Captain Garro escaped Istvaan III aboard the damaged Imperial frigate Eisenstein and fled to Terra to warn the Emperor that Horus had turned Traitor.

On Istvaan III, the remaining Loyalists, under the command of Captains Tarvitz, Garviel Loken and Tarik Torgaddon, another Loyalist member of the Sons of Horus, fought bravely against their own traitorous brethren. Yet, despite some early successes that delayed Horus' plans for three full months while the battle on Istvaan III played out, their cause was ultimately doomed by their lack of air support and Titan firepower. During the battle the Sons of Horus Captains Ezekyle Abaddon and Horus Aximand were sent to confront their former Mournival brothers, Loken and Torgaddon. Horus Aximand beheaded Torgaddon, but Abaddon failed to kill Loken when the building they were in collapsed. Loken survived and witnessed the final orbital bombardment of Istvaan III that ended the Loyalists' desperate defence. To prove his worth and loyalty to Lord Commander Eidolon of the Emperor's Children -- and thus to his Primarch, Fulgrim -- Captain Lucius of the 13th Company of the Emperor's Children, the future Champion of Slaanesh known as Lucius the Eternal, turned against the Loyalists that he had fought beside because of his prior friendship with Saul Tarvitz. Lucius slew many of them personally, an act for which he was then accepted back into the Emperor's Children on the side of the Traitors. In the end, the Loyalists retreated to their last bastion of defense, only a few hundred of their number remaining. Finally, tired of the conflict, Horus ordered his men to withdraw, and then had the remains of the Choral City bombarded into dust for a final time from orbit.

Flight of the Eisenstein

"His dreams and hope are in ruin, his trust proved false, and his brightest son fallen to darkness. These are bitter tidings, a nightmare made flesh, but they must reach the Emperor."

— Battle-Captain Nathaniel Garro, Legiones Astartes Death Guard

Seventy Loyalists of the Death Guard Legion and the Imperial Saint Euphrati Keeler led by Battle-Captain Garro had commandeered the Imperial frigate Eisenstein and, evading the Traitor forces of Horus, were able to escape from the Istvaan System into the Immaterium, after being told what was happening on the planet. The Eisenstein was badly damaged by the Death Guard battleship Terminus Est during its escape from Istvaan III and it was assaulted by undead minions, including the first known Plague Marines of Nurgle while it was within the Immaterium. This assault by Warp entities forced the ship to make a crash emergence from the Warp. The repeated traumas left all of the frigate's astropaths dead, and its lone Navigator was mortally wounded. However, Garro managed to attract the attention of passing Loyalist starships by setting the vessel's Warp-Drives to self-destruct and ejecting them from the starship. Rogal Dorn's Imperial Fists Legion's massive mobile fortress-monastery Phalanx and the Legion's fleet had been becalmed in the Warp for some time due to the waxing power of the Ruinous Powers as the Heresy began, and his Navigators sensed the detonation of the Eisenstein's Warp-Drives. Charting an immediate course for the location of the detonation, Dorn met with Garro, who explained to him all that had happened with the Traitor Legions. Dorn was reluctant to believe Garro's tale, but overwhelming proof from a Remembrancer (journalist) named Mersadie Oliton who had escaped from Horus' flagship, the Vengeful Spirit, and Garro's dogged insistence finally convinced the Primarch. The Phalanx fortress set a course for Terra.

The fate of the Eisenstein's survivors is unknown. Sequestered on Luna in a tower belonging to the Sisters of Silence after their arrival in-system, Garro, the rest of his Loyalist Death Guard, Captain Iacton Qruze of the Luna Wolves, and the warriors of the Silent Sisterhood faced one of the Death Guard Marines named Solun Decius who had succumbed to the temptations of the Chaos God Nurgle after being infected with one of the Lord of Plague's more virulent illnesses now known as Nurgle's Rot by the undead minions Nurgle had unleashed on the Eisenstein. Seeking to end his pain, Decius allowed himself to become possessed by a Lesser Daemon of Nurgle, a variant Plaguebearer called the Lord of the Flies. Afterwards, Garro and Captain Qruze were met by Malcador the Sigillite, leader of the Council of Terra and the Emperor's Regent, who informed them that the Emperor had need of people who were strong of will and as "inquisitive" as they. Malcador ordered Garro to begin to comb the galaxy for 7 other Astartes from both the Traitor and Loyalist Legions who with him would become the founding members of the Imperial Inquisition and the militant arm of its daemon-hunting Ordo Malleus -- the Grey Knights.

Preparations and Allegiances

Much of Horus' later success arose from the thorough groundwork he had laid before the opening shots of the Heresy were fired at Istvaan III. He had already swayed the Primarchs Angron and Mortarion, of the World Eaters and Death Guard Legions, respectively, to the side of Chaos because of their own various personal grudges against the Emperor. Fulgrim of the Emperor's Children had been lured to the side of the Warmaster by the promise of power and personal perfection that the Chaos Gods, especially Slaanesh, offered to him and his vain Astartes. Lorgar of the Word Bearers, who had been responsible for the nascent rebellion and Horus's own corruption by Chaos, was also with the Warmaster. Three of the most loyal Legions who could not be swayed to the side of Chaos, the Dark Angels, Blood Angels and Ultramarines and their Primarchs, were sent on missions by the Warmaster far from Terra and the Istvaan System. The Imperial Fists and White Scars were too close to Terra to be contacted without raising suspicion, though Horus believed -- mistakenly -- that the White Scars' Primarch Jaghatai Khan, would ultimately take his side. Shortly before the Drop Site Massacre on Istvaan V, Fulgrim also attempted to sway his friend Ferrus Manus of the Iron Hands Legion to Horus' cause by using many of the same inducements that had been offered to the Adeptus Mechanicus, with whom the Iron Hands were closely allied in both temperament and philosophy. This attempt failed, and Fulgrim barely escaped with his life. Angered by the rebuff, Fulgrim promised he would deliver Manus' severed head to Horus in recompense, a promise he kept on Istvaan V. The Blood Angels were sent to the daemon-infested Signis Cluster and the Ultramarines to the world of Calth, where a large Word Bearers force, under First Captain Kor Phaeron, had massed to hold Roboute Guilliman's equally massive Legion in place while Horus made his play for Terra.

Of the other eventual Traitor Primarchs, Konrad Curze, the Night Haunter, was due to face disciplinary action from the Emperor which he did not believe he deserved; the Alpha Legion Primarch Alpharius had always been closer personally to his brother Horus than to his father the Emperor, although some evidence indicates that he and his twin brother Omegon's turn to Chaos was driven by mistaken loyalty to the Emperor; and the Iron Warriors' Primarch Perturabo's open and bitter rivalry with Rogal Dorn of the Imperial Fists and his feeling that he and his Legion were handed the worst tasks in the Great Crusade for which they never received the recognition they believed they were due made him an easy target for corruption.

The Thousand Sons had never planned to join Horus, but the path Tzeentch had mapped for that Legion and their potent psychic Primarch Magnus the Red ultimately led them to Chaos regardless. Unfortunately, the Space Wolves' unexpected assault on the Thousand Sons' homeworld -- a brutal campaign remembered as the Scouring of Prospero -- resulted in the destruction of the libraries of precious knowledge that Magnus and his fellow Thousand Sons held so dear. Mortally wounded by Leman Russ, Magnus fell to temptation as he watched Tizca, the capital city of Prospero and its famed libraries of ancient knowledge burn and he called out to the Chaos God Tzeentch to save both himself and the remains of his Legion. The God of Sorcery was only too happy to oblige and he transported Magnus and the Thousand Sons through the Warp to the Daemon World later known as the Planet of the Sorcerers. Magnus became a Daemon Prince of Tzeentch and now desired only vengeance against the Emperor for what he saw as a betrayal, never realizing that it was Horus who had truly engineered his downfall and corruption.

The remaining Space Marine Legions -- the Raven Guard, Salamanders, Iron Hands and Space Wolves -- remained staunchly loyal to the Emperor, though all but the Space Wolves would pay dearly for it in the battles to come. Beyond the Legions, Horus had already swayed Magos Regulus of the Adeptus Mechanicus to his side with promises of the Standard Template Construct (STC) databases of ancient technology recovered during the war with the Auretian Technocracy. This alliance delivered crucial Adeptus Mechanicus and Titan support to the Warmaster's Traitor Legion and Traitor Imperial Army forces.

Meanwhile, Horus and his confidante and mentor in the ways of Chaos, the Word Bearers' First Chaplain Erebus, conducted a ritual designed to communicate with the Chaotic entities of the Warp. They established contact with a daemon called Sarr'kell, who acted as an emissary of the Chaos Gods to Horus and the Traitor Legions. Horus was then deceived by the daemon into believing that the Chaos Gods had no interest in dominating the material universe, and were only lending their support so that Horus could overthrow the Emperor, who they claimed was creating devices that could destroy the daemonic beings of the Immaterium. Horus agreed, and promised to swear loyalty to Chaos Undivided after his fateful operations on Istvaan III.

Drop Site Massacre on Istvaan V

"It was treachery at first. To turn against brothers, to kill for personal advancement and power. But we have seen them, how their minds and bodies have been corrupted. Their very belief systems have been warped. This is no longer Horus's treachery. It is his heresy."

— Attributed to Roboute Guilliman, Lord of Ultramar and Primarch of the Ultramarines Legion

The Istvaan System’s third world, comfortably close enough to the sun to support human life, was now a virus-soaked mass grave marking the anger of Horus Lupercal. The world’s population was nothing more than contaminated ash scattered over lifeless continents, while the bones of their cities remained as blackened smears of burnt stone – a civilisation reduced to memory in a single day. The orbital bombardment from the Warmaster’s fleet, payloads of incendiary shells and virus-laden biological warfare pods, had seemingly spared nothing and no one anywhere in the world. Istvaan III lingered now in silent orbit around its sun, almost grand in the extent of its absolute devastation, serving as the scarred tombstone for the death of an empire.

Ringing Istvaan V was one of the largest fleets ever gathered in the history of the human species. Without a doubt, it was the most impressive coalition of Astartes vessels, with the scouts, cruisers, destroyers and command ships of seven entire Legions. With a precision that required mass calculation, the fleets of seven Astartes Legions hung in the skies above Istvaan V. Shuttles and gunships ferried between the heaviest cruisers, while the decks of every warship made ready to deploy their warriors in an unprecedented, unified planetfall. Horus, traitorous son of the Emperor, was making his stand on the surface. The Imperium of Man had sent seven Legions to kill its wayward scion, little knowing four of them had already spat on their oaths of allegiance to the Throneworld.

Aboard the Fidelitas Lex, Lorgar's flagship played host to a gathering of rare significance. There were commanders from the Night Lords, Alpha Legion, Iron Warriors as well as three additional Primarchs; Nighthaunter, Alpharius Omegon and Perturabo. Lorgar strode to the centre of the gathering of Traitors. He then proceeded to impress upon the gathering of his sons, brothers and cousin Astartes of the importance of their cause, and of the significance this day would hold in history. The Word Bearers and their allies believed that the Imperium had failed them and by being flawed to its core, imperfect in its pursuit of a perfect culture, and in its weakness against the encroachment of xenos breeds that sought to twist humanity to alien ends. And it had failed them, most of all, by being founded upon lies. The Imperium was forged by a dangerous deceit, and had eroded them all by demanding they sacrifice truth on the altar of necessity. This was an empire, propagated by sin, that deserves to die. And here, on Istvaan V, they would begin the purge. From the ashes would rise the new kingdom of mankind: an Imperium of justice, faith and enlightenment. An Imperium heralded, commanded and protected by the avatars of the gods themselves. An empire strong enough to stand through a future of blood and fire. The Emperor believes them loyal. Their four Legions were ordered to Istvaan V on His misguided conviction alone. But their coalition here and now was the fruit of decades’ worth of planning. It was ordained, and brought about according to ancient prophecy. No more hiding in the shadows. No more manipulating fleet movements and falsifying expeditionary data. From this day forward, the Alpha Legion, the Word Bearers, the Iron Warriors and the Night Lords would stand together – bloodied but unbowed beneath the flag of Warmaster Horus, the second Emperor. The true Emperor. First Captain Sevatar of the Night Lords Legion uttered, "Death to the False Emperor," becoming the first living soul to utter the words that would echo through the millennia. The curse was taken up by other voices, and soon it was being cried in full-throated roars.

"Death to the False Emperor! Death to the False Emperor! Death! Death! Death!"

Thousands of Drop Pods and Stormbirds were deployed for the initial assault. The first wave was under the overall command of the Primarch Ferrus Manus and besides his own X Legion, the Salamanders led by Vulkan, and the Raven Guard under the command of their Primarch Corax joined him. Vulkan's Legion assaulted the left flank of the Traitors' battle line while Ferrus Manus, the Iron Hands' First Captain Gabriel Santor, and 10 full companies of elite Morlocks Terminators charged straight into the centre of the enemy lines. Meanwhile, Corax's Legion hit the right flank of the enemy's position. The odds were considered equal; 30,000 Traitor Marines against 40,000 Loyalists. Horus was aware of the location of the Loyalists' chosen drop site and his troops fell upon the Loyalist Legions.

The battlefield of Istvaan V was a slaughterhouse of epic proportions. Treacherous warriors twisted by hatred fought their former brothers-in-arms in a conflict unparalleled in its bitterness. The mighty Titan war engines of the Machine God walked the planet’s surface and death followed in their wake. The blood of heroes and traitors flowed in rivers, and the hooded Hereteks Adepts of the Dark Mechanicum unleashed perversions of ancient technology stolen from the Auretian Technocracy to wreak bloody havoc amongst the Loyalists. All across the Urgall Depression, hundreds died with every passing second, the promise of inevitable death a pall of darkness that hung over every warrior. The Traitor forces held, but their line was bending beneath the fury of the first Loyalist assault. It would take only the smallest twists of fate for it to break. The forces on the surface had been embattled for almost three solar hours with no clear victor emerging. The Loyalists waited for their second wave of "allies" to make planetfall, believing they would be reinforced for their final advance. In truth, the Traitors all knew their parts to play in this performance. They were all aware of the blood they would shed to spare their species from what they believed to be its coming destruction, and install Horus as the new Master of Mankind.

Though the Iron Hands, Raven Guard and Salamanders had managed to make a full combat drop and secured the drop site, known as the Urgall Depression, they did so at a heavy cost. Overwhelmed with rage, the headstrong Ferrus Manus disregarded the counsel of his brothers Corax and Vulkan and hurled himself against the fleeing rebels, seeking to bring Fulgrim to personal combat. His veteran troops -- comprising the majority of the X Legion's Terminators and Dreadnoughts -- followed. What had begun as a massed strike against the Traitors' position was rapidly turning into one of the largest engagements of the entire Great Crusade. All told, over 60,000 Astartes warriors clashed on the dusky plains of Istvaan V. For all the wrong reasons, this battle would go down in the annals of Imperial history as one of the most epic confrontations ever fought.

The Urgall Depression was churned to ruination beneath the boots and tank treads of countless thousands of Astartes warriors and their Legions' armour divisions. The loyal Primarchs could be found where the fighting was thickest: Corax of the Raven Guard, borne aloft on black wings bound to a fire-breathing Jump Pack; Ferrus Manus of the Iron Hands at the heart of the battlefield, his silver hands crushing any Traitors that came within reach, while he pursued and dragged back those who sought to withdraw; and lastly, Vulkan of the Salamanders, armoured in overlapping Artificer Armour, thunder clapping from his warhammer as it pounded into yielding ceramite armour, shattering it like porcelain.

The traitorous Primarchs slew in mirror image to their brothers: Angron of the World Eaters hewing with wild abandon as he raked his chainblades left and right, barely cognizant of who fell before him; Fulgrim of the lamentably-named Emperor's Children, laughing as he deflected the clumsy sweeps of Iron Hands warriors, never stopping in his graceful movements for even a moment; Mortarion of the Death Guard, in disgusting echo of ancient Terran myth, harvesting life with each reaving sweep of his great war scythe.

And Horus, Warmaster of the Imperium, the brightest star and greatest of the Emperor's sons. He stood watching the destruction while his Legions took to the field, their liege lord content in his fortress rising from the far edge of the ravine. Shielded and unseen by his brothers still waging war in the Emperor's name. At last, above this maelstrom of grinding ceramite, booming tank cannons and chattering bolters -- the gunships, Drop Pods and assault landers of the second wave burned through the atmosphere on screaming thrusters. The sky fell dark with the weak sun eclipsed by ten thousand avian shadows, and the cheering roar sent up by the Loyalists was loud enough to shake the air itself. The Traitors, the bloodied and battered Legions loyal to Horus, fell into a fighting withdrawal without hesitation.

The second wave of "Loyalist" Space Marine Legions descended upon the landing zone on the northern edge of the Urgall Depression. Hundreds of Stormbirds and Thunderhawks roared towards the surface, their armoured hulls gleaming as the power of another four Astartes Legions arrived on Istvaan V. Yet the Space Marine Legions of the reserve were no longer loyal to the Emperor, having already secretly sworn themselves to Chaos and the cause of Horus. The Night Lords of Konrad Curze, the Iron Warriors of Perturabo, the Word Bearers of Lorgar Aurelian, and the Alpha Legion of Alpharius represented a force larger than that which had first begun the assault on Istvaan V. The secret Traitor Legions mustered in the landing zone, armed and ready for battle, unbloodied and fresh.

The Iron Warriors had claimed the highest ground, taking the loyalist landing site with all the appearance of reinforcing it through the erection of prefabricated plasteel bunkers. Bulk landers dropped the battlefield architecture: dense metal frames fell from the cargo claws of carrier ships at low altitude, and as the platforms crashed and embedded themselves in the ground, the craftsmen-warriors of the IV Legion worked, affixed, bolted and constructed them into hastily-rising firebases. Turrets rose from their protective housing in the hundreds, while hordes of lobotomised servitors trundled from the holds of Iron Warriors troopships, single-minded in their intent to link with the weapons systems’ interfaces. The Word Bearers bolstered their brother Legions on one flank of the Urgall Depression while the Night Lords took positions on the opposite side. Down the line, past the mounting masses of Iron Warriors battle tanks and assembling Astartes, First Captain Sevatar of the Night Lords and his First Company elite, the Atramentar took up defensive positions. Both the Word Bearers and the Night Lords were to be the anvil, while the Iron Warriors would be the hammer yet to fall. The enemy would stagger back to them, exhausted, clutching empty bolters and broken blades, believing their presence to be a reprieve.

Dragging their wounded and dead behind them, Corax and Vulkan led their forces back to the drop site to regroup and to allow the warriors of their recently arrived brother Primarchs of the second wave a measure of the glory in defeating Horus. Though they voxed hails requesting medical aid and supply, the line of Astartes atop the northern ridge remained grimly silent as the exhausted warriors of the Raven Guard and Salamanders came to within a hundred metres of their allies. It was then that Horus revealed his perfidy and sprung his lethal trap. Inside the black fortress where Horus had made his lair, a lone flare shot skyward, exploding in a hellish red glow that lit the battlefield below. The fire of betrayal roared from the barrels of a thousand guns, as the second wave of Astartes revealed where their true loyalties now lay. Ferrus Manus looked on in stunned horror as Fulgrim laughed at the look on his brother's face as the forces of his "allies" opened fire upon the Salamanders and Raven Guard, killing hundreds in the fury of the first few moments, hundreds more in the seconds following, as volley after volley of Bolter fire and missiles scythed through their unsuspecting ranks. Even as terrifying carnage was being wreaked upon the Loyalists below, the retreating forces of the Warmaster turned and brought their weapons to bear on the enemy warriors within their midst. Hundreds of World Eaters, Sons of Horus and the Death Guard fell upon the veteran companies of the Iron Hands, and though the warriors of the X Legion continued to fight gallantly, they were hopelessly outnumbered and would soon be hacked to pieces. The Iron Hands had damned themselves by remaining in the field.

The Raven Guard front ranks went down as if scythed, harvested in a spilling line of detonating bolter shells, shattered armour and puffs of bloody mist. Black-armoured Astartes tumbled to their hands and knees, only to be cut down by the sustained volley, finishing those who fell beneath the initial storm of head- and chest-shots. Seconds after the first chatter of bolters, beams of achingly bright laser slashed from behind the Word Bearers as the cannon mounts of Land Raiders, Predators and defensive bastion turrets gouged through the Raven Guard and the ground they stood upon. The Iron Warriors and Word Bearers kept reloading, opening fire again, hurling grenades and prepared to fall back. The Word Bearers Legion had taken up landing positions on the west of the field, ready to sweep down and engage the Raven Guard from the flank.

The Raven Guard were confronted by the treacherous Word Bearers, with their Primarch Lorgar, the First Captain Kor Phaeron and the First Chaplain Erebus at their vanguard. The two Legions fought one another in bitter combat. In the midst of this battle, the Word Bearers unleashed the elite unit known as the Gal Vorbak -- Astartes who had allowed themselves to be possessed by daemons. They attacked the Raven Guard's Primarch en masse, but despite the advantage of their numbers, Corax's formidable abilities as a consummate warrior proved to be more than a match for the possessed Astartes, and he slew them with impunity. Seeing the slaughter of his most favoured sons, Lorgar intervened and prevented the death of the remaining Gal Vorbak Astartes. The two opposing Primarchs then duelled one another in close combat, and the Raven Guard's Primarch quickly gained the upper hand over his outmatched brother. Lorgar had always been more of a scholar than a warrior and Corax prepared to execute him for his betrayal of the Emperor. Lorgar was spared from execution by the intervention of the Night Lords' Primarch, Konrad Curze, at the last moment. The Night Haunter and the Raven fought a brutal melee. Curze quickly gained the upper hand over his battle-weary brother and prepared to slay him, but Corax managed to escape death by taking to the sky with his master-crafted Jump Pack.

The outnumbered Loyalists were then surrounded and brutally butchered. Refusing to surrender, the remaining Raven Guard and Salamanders Astartes stubbornly defended themselves, trying to hold off the inevitable slaughter for as long as possible. Though they suffered an atrocious number of casualties, the Loyalists managed to hold their own, until the Primarchs Mortarion of the Death Guard and Angron of the World Eaters joined the fray. Bolstered by the support of the infamous Imperator-class Titan Dies Irae, the Traitors killed tens of thousands of Loyalist Astartes. At the height of the massacre the Warmaster Horus entered the fray, at the head of the elite Sons of Horus Terminators known as the Justaerin, slaughtering the Loyalists in wrathful anger.

Any hope for escape for the Loyalists was quickly crushed when the traitorous Iron Warriors destroyed the first wave's drop ships. The Loyalist starships still orbiting the embattled planet were also largely annihilated by the vastly superior numbers of the Traitor's fleet. Despite the odds arrayed against them, some of the Loyalists on the ground managed to survive against these odds—they miraculously escaped through the tightening cordon of Traitors that surrounded their position. The Raven Guard fared better than the Salamanders in escaping the brutal massacre. But the Salamanders managed to assist a few surviving Astartes from the decimated Iron Hands Legion to also escape the slaughter. Imperial history does not record the fate of these surviving Salamanders or their missing Primarch Vulkan. The Raven Guard's Primarch just barely managed to board a fleeing Thunderhawk gunship to make good his escape, but was thwarted in the attempt when it was shot down almost immediately by the gunfire of the Traitors. The badly damaged ship crashed on the outskirts of the Urgall Plateau.

Raven's Flight

Corax had survived the crash and quickly ordered the remaining warriors of his Legion to regroup. He learned to his shock that a large percentage of his Legion had been utterly annihilated during the ensuing slaughter. They took to the highlands of the surrounding hills and took to the shadows, hiding from their relentless pursuers. During their flight, their position was nearly discovered by a roving armour column of traitorous Iron Warriors, but they were destroyed in a Raven Guard ambush and wiped out before they could report what they had learned.

Thirty days after the initial planetary assault, the future looked grim for the fleeing Raven Guard survivors. They had received no word from either the surviving Iron Hands or Salamanders Legions. Corax ordered his warriors to dig-in and hold position at Lurgan Ridge while he undertook a lone reconnaissance of their original drop site to determine their options. Utilising his innate psi-abilities to escape detection, Corax successfully conducted a reconnaissance of the Traitor Legions' positions around the heavily fortified drop site. Though the Primarch informed his men that his mission was to reconnoiter the drop site, his primary objective was to scour the Urgall Plateau for the bodies of his fallen sons, but he failed to find them.

After 98 days of relentless pursuit, the Raven Guard survivors were finally backed into a literal corner. Caught upon the windswept mountainside, Corax's Legion remained resolute. Behind the peak stretched the great salt plains that had forced them into this last, defiant stand. Ahead of them massed the might of the World Eaters, the rage-driven Astartes Legion of Angron, who strode at their head roaring for the blood of his brother. A sea of white and blue World Eaters Astartes spattered with the red of gore swept up from the valley intent on the destruction of the Raven Guard. Maddened by their neural implants and driven into a battle-frenzy by inhuman cocktails of stimulants, the berserk warriors of the World Eaters pounded up the sloping mountainside while their tanks and guns provided covering fire; every warrior bellowed his eagerness to fulfill the blood oaths he had sworn to his Primarch.

But before they could utterly eradicate the surviving Raven Guard Astartes, the World Eaters were attacked from an unexpected quarter. Broad-winged aircraft plunged down from the scattering of clouds, missile pods rippling with fire. A swathe of detonations cut through the ranks of the World Eaters, ripping through their advance companies. Incendiary bombs blossomed in the heart of the approaching army, scattering white-hot Promethium over the steep slopes. Corax looked on with incredulity as blistering pulses of plasma descended from orbit, cutting great gouges into Angron’s Legion.

The roar of jets became deafening as drop ships descended on pillars of fire: black drop ships emblazoned with the badge of the Raven Guard. The Legionaries scattered to give the landing craft space to make planetfall. As soon as their thick hydraulic legs touched the ground, their ramps whined down and boarding gateways opened. The Raven Guard met their rescuers in stunned disbelief. These drop ships were part of a desperate rescue mission that had been devised by Commander Branne, a Raven Guard Captain who had been left in charge of the Legion’s homeworld of Deliverance. Without further delay the Raven Guard survivors quickly prepared for embarkation and escaped aboard the drop ships, breaking for orbit and leaving behind the frothing berserkers of the World Eaters, their angry Primarch futilely baying for blood.

Horus Triumphant

"The road to Terra is open. The time has come for us to take the war to the Emperor in his most impregnable fastness! We will make immediate preparation for the invasion of Terra and an assault on the Imperial Palace. Make no mistake, and it will be ours, my brothers! This will be no easy task, for the Emperor and his deluded followers will fight hard to prevent us from interfering with his plans for godhood. Doubtless much blood has yet to be spilled, theirs and our own, but the prize is the galaxy itself...Are you with me?"

— Warmaster Horus, Master of Istvaan

After the killing had stopped and the dead were gathered into great funeral pyres across the broken desert of the Urgall Depression, the once-grey skies of the planet burned orange with the reflected glow of a thousand pyres. The firelight bathed the rippling, glassy sands in a warm radiance, and towering pillars of black smoke from the burning corpses filled the air. Thousands of Astartes loyal to Horus gathered before a great reviewing stand, constructed by the Tech-priests of the Dark Mechanicus with astonishing speed. As the sun began to sink beyond the horizon, the smooth black planes of the stand shone with a blood red glow. The stand was erected as a series of cylinders of ever decreasing diameter, one standing atop another. The base was perhaps a thousand metres in width, constructed as a great grandstand upon which the Sons of Horus stood, their pre-eminent position as the elite of the Warmaster in no doubt after this great victory. Each warrior bore a flaming brand, and the firelight cast brilliant reflections from their armour.

Atop this pedestal of flame was another platform, occupied by the senior officers of the XVI Legion. Above the senior officers of the Sons of Horus stood the Traitor Primarchs. The sheer magnificence of such a gathering of might was breathtaking. Seven beings of monumental power stood on the penultimate tier of the reviewing stand, their armour still stained with the blood of their foes, their cloaks billowing in the winds that swept the Urgall Depression. Finally, the uppermost tier of the reviewing stand was a tall cylinder of crimson that stood a hundred metres above the Primarchs. Horus stood on top of it, his clawed gauntlets raised in salute. A furred cloak of some great beast hung from his shoulders, and the light of the corpse pyres reflected from the amber eye upon his breastplate. The Warmaster was illuminated from below by a hidden light source, bathing him in a red glow that gave him the appearance of the statue of a legendary hero, as he stood looking down on the endless sea of his followers from the towering platform.

As the sun finally dipped below the horizon, a flight of assault craft roared over the Urgall Hills, their wings dipping in salute to the mighty warrior below. Solid waves of cheering crashed against the reviewing stand, howls of adulation torn from tens of thousands of throats. No sooner had the aircraft passed overhead than the massed Astartes began to march around the reviewing stand, their arms snapping out and hammering their breastplates in salute of the Warmaster. At some unseen signal a flame ignited on the northern slopes of the Urgall Depression and a blazing line of phosphor leapt across the ground in a snaking arc that described the outline of an enormous blazing eye upon the hillside. The adulation soared to new heights as the Eye of Horus seared itself into the sands of Isstvan V, the Warmaster’s forces roaring themselves hoarse in his praise. Super-heavy tanks fired in salute of Horus, and the towering immensity of the Dies Irae inclined its massive head in a gesture of respect. The ashes of the dead fell like confetti over Horus' mighty army as thousands of Traitor Astartes cheered, their cries of "Hail Horus! Hail Horus!" resounding long into the darkness.

Barely a handful of Loyalist Space Marines escaped with their lives from Istvaan V to bring dreadful word of the further betrayal of four more Space Marine Legions to the Emperor. A critically wounded Corax made the dangerous journey through the Immaterium back to Terra, arriving 133 days after departing the Istvaan System and finally reaching the Sol System -- the heart of the Imperium -- to seek audience with the Emperor. Vulkan was missing and presumed dead, though he would later reemerge after a harrowing journey back to Terra himself, to lead his Legion once more. The Salamanders, along with the Iron Hands and the Raven Guard, would spend the remainder of the Horus Heresy rebuilding their decimated Legions and were too weakened to play any further role in the great conflict.

In the days after the battle, the Traitor Legions salvaged a large number of vehicles, wargear and other war materiel from what the Loyalist Legions had left on the field. This salvage was repaired and modified for the Traitor Legions' use and then put back into frontline service to be used against the Imperium. Some of this equipment would still be in service with certain Chaos Space Marine warbands in the late 41st Millennium. Orbital space around Istvaan V was busy as the vessels of 8 Legions assumed formation prior to transit to the system jump point. Over 3,000 vessels jostled for position above the darkened fifth planet, their holds bursting with warriors sworn to the service of Horus. Tanks and monstrous war machines had been lifted from the planet with incredible efficiency and an armada greater than any in the history of the Great Crusade assembled to take the fire of war into the very heart of the Imperium.

In the days after the Drop Site Massacre, Horus called for a conclave of the Primarchs of all 8 of the Traitor Legions aboard his flagship, the Vengeful Spirit. Five of the Primarchs, including four who had fought at Istvaan V, met in person, including Horus, Fulgrim, Angron, Mortarion and Lorgar. Three appeared through the use of hololithic emitters that transmitted their signals through the Warp, including Perturabo, Night Haunter and Magnus the Red, who had only recently joined the Traitors after the Scouring of Prospero when the broken remains of his XV Legion had been transported by Tzeentch into the Eye of Terror to the Planet of the Sorcerers. The Thousand Sons, bitter at what they perceived as their betrayal by the Emperor, now willingly became the eighth Traitor Legion. The council of Traitor Primarchs made their plans for the next step in their war against the Emperor and then each Legion went its way according to its assigned role.

The fleets of Angron, Fulgrim, Mortarion, Lorgar and Horus' own Legion would rendezvous at Mars, now that word had come from the Tech-priest Regulus, the Mechanicus' liaison with the 63rd Expeditionary Fleet, of that planet’s fall to Horus’ supporters within the Mechanicus during the internecine conflict known as the Schism of Mars. With the manufacturing facilities of Mondus Gamma and Mondus Occullum wrested from the control of the Emperor’s forces, the forges of Mars were free to supply the Warmaster’s army. The eager warriors of the Alpha Legion were singled out by Horus for a vital mission, one upon which the success of the entire venture could depend. Following Horus' manipulation of Leman Russ into assaulting the homeworld of the Thousand Sons, the Space Wolves were known to be operating in the region of Prospero. In the nearby system of Chondax, the White Scars of Jaghatai Khan were sure to have received word of Horus’ rebellion and would no doubt attempt to link up with the Space Wolves. Horus could not allow such a grave threat to appear, and so the warriors of Alpharius were to seek out and attack these Legions before they could join forces.

The Night Haunter’s fleet had already departed, bound for the planet of Tsagualsa, a remote world in the Eastern Fringe that lay shrouded in the shadow of a great asteroid belt. From there, the Night Lords’ terror troops would begin a campaign of genocide against the Imperial strongholds of Heroldar and Thramas, star systems that, if not taken, would leave the flanks of the Warmaster’s strike on Terra vulnerable to attack. The Thramas System was of particular importance, as it comprised a number of Mechanicus Forge Worlds whose loyalty was still to the Emperor. This campaign would also serve to tie up the dreaded Dark Angels Legion, so that the forces of the Lion wouldn't be brought to bare against Horus and his upcoming campaign against Terra.

The ships of the Iron Warriors prepared to make the journey to the Phall System where a large fleet of Imperial Fists vessels were known to be regrouping after a failed attempt to reach Istvaan V in time to join the Loyalist assault. Though Rogal Dorn’s warriors had played no part in the Drop Site Massacre, Horus could not allow such a powerful Loyalist force to remain unmolested. The enmity between bitter Perturabo and proud Dorn was well known, and it was with great relish that the Iron Warriors set off to do battle with their old rivals. With his flanks covered and the Space Marine forces that could potentially reinforce the heart of the Imperium soon to be embroiled in war, the Traitors were ready to unleash 7 Terran years of devastating civil war upon the Imperium in the name of Horus and the Dark Gods.

Disposition of the Traitor Legions

Much remains uncertain in regards to the strategic positions, deployments and configuration of the forces loyal to the Warmaster Horus in the early phases of the war, and even their full scope and extent cannot be ascertained with any genuine certainty by the historian. Perhaps a full half of the Titan Legions and the numberless hosts of the Excertus Imperialis -- hundreds of millions of soldiers, vessels and war machines -- had either through corruption, misguided loyalty to Horus or simple blind ignorance in compliance with their orders, sided with the Warmaster. But this alone would never be enough to overthrow the Emperor. A thousand Battleships might be wrecked against the defences of Terra, ten million Auxilia might spend their lives in besieging the Eternity Gate, and ten millions follow them, and again and again, but Horus knew this alone would never avail him. Not against the superhuman warriors who held it; not against the Legio Custodes and defences designed by the Emperor's own hand and garrisoned now by Rogal Dorn and his sons, the Imperial Fists. So it was that the greatest attention should be placed on the location and strengths of the Traitor Legions. The Legions at the Warmaster's side during this period at the start of the conflict comprised eight Legions. The shattered remnants of the Thousand Sons, who would later join the Traitor's cause in earnest, were not yet at this time fully active in the war, and were still largely of unknown disposition and even allegiance following the apocalyptic Battle of Prospero. Although the Drop Site Massacre had inflicted terrible wounds on the Loyalist forces involved, the Traitor Legions had not themselves come away from the cataclysm unscathed which, combined with losses suffered during the recent purging of the Traitors' ranks at Istvaan III and elsewhere, had weakened Horus' position from its notional strength when the die for war was cast.

Hard facts regarding the operational Legiones Astartes numbers are impossible to ascertain, but credible estimates place Traitor losses during the battles of the Istvaan, Phall and Paramar Systems in the region of 100,000 Space Marine fatalities, compared to an unknown number, perhaps three or four times as high, as a death toll for those who remained loyal to the Emperor. This, by many estimates, left something in the region of 900,000 Legiones Astartes under arms in the Warmaster's cause, with perhaps two thirds or more of that figure in the Loyalist camp. This estimate, however, was still far from certain, one which is itself further distorted by the events that were to shortly unfold on Calth and Signus Prime. From this calculation -- as equivocal as it is -- it is possible to determine that even after the Drop Site Massacre, the military advantage Horus had yet gained was simply not large enough in scale to make an immediate and direct assault on Terra and the Imperial heartlands of the Segmentum Solar, roused as they were now against attack, a strategically viable option, let alone one that would guarantee victory. This was the reality that Horus faced in determining his next move.

The Traitor Legions were the single greatest power and strength of Horus' armies, but they were far from alone. To achieve the ultimate aim of laying siege to Terra and casting down the Emperor, the Traitors would have to call upon and fight alongside a wide array of other forces, some simply obeying the will of masters of long association, while others were brought into the Warmaster's fold through bribery, pacts of alliance, coercion or the infamous progress of the Dark Compliance. As early as the Traitors' defence of the Urgall Depression during the Istvaan V Drop Site Massacre, mortal Auxilia troops were used with unspeakable callousness by their Space Marine masters, herded towards the enemy as bullet-soaks on which the foe's ammunition would be expended before battle was truly joined, and such disregard for the purely human component of the Warmaster's forces would be shown time and again throughout the bloody years that were to follow. This was not however universal, as at other times, Horus and his principal subordinates entrusted vital missions to the more reliable of his mortal allies, and split entire battlegroups of Traitor Auxilia and Armada units on independent operations either of conquest or destruction, although it was not uncommon for him to assign an officer or cadre of the Sons of Horus, Iron Warriors or Alpha Legion to oversee such forces in the field.

The Traitor Auxilia

"Fear not that I am come to demand your surrender. There is in me no such gift of mercy. I am come to deliver not words, but fire."

— Baron Armelan, Emissary of the Warmaster to the planetary council of Subinus

By far the largest numbers of human troops under the Warmaster's command were drawn from the Imperial Auxilia, and ranged from the massive Imperial Army Regiments of the Line to the elite ranks of the Solar Auxilia. In addition to these experienced and well-equipped forces, the Warmaster also exercised command over native militias, planetary tithe-hosts and Rogue Trader Conquestor companies. The latter were able to range far and wide, often serving as emissaries of the Warmaster 