As the Yugoloths grow stronger, they wish to have the same neutrality that their creators, the Baernaloths, have and their most powerful general, known as the General of Gehenna, created a magical stone, the Heart of Darkness, that drives the impurities of Law and Chaos out of anyone who touches it. Summoning all of the Yugoloths, they touch the Heart and the General channels the expunged forces onto separate planes of existence - the forces of law became larvae on Baator and the forces of chaos became larvae on the Abyss.

After eons, the larva began growing and evolving on their respective planes and they began exploring not only their plane but the planes near them which include Gehenna and Carceri. These larvae grew into the Devils and Demons, though there were originally known as the Baatezu and the Tanar’ri respectively.

Now is when the Yugoloths are discovering the power of their neutrality and the Baernaloths, their creators, suddenly withdraw into the Gray Waste, leaving behind little evidence of their existence.

The Start of the Blood War

Once the Devils and Demons gained control of their planes, they each decided at the same time to begin investigating what else was out there. Scouts were sent by both sides and each found the Yugoloths in the adjacent planes. The Devil entourage saw the creatures as weak-willed and without any sort of ethical guidance, they then left the Yugoloths and continued eventually finding the Abyss. In the Abyss, they first meet the chaotic Demons and realize that they have such a different philosophical ideology that it manifests as violent hatred and the Devils begin slaughtering as many Demons as they can find before returning to Baator to report what they found.

During this time, the Demons have sent their patrols out. They find the Yugoloths and begin torturing and killing them for fun before heading deeper into the planes and eventually finding Baator. Here they first meet the Devils and begin murdering and destroying all they can find. Many decide to continue through Baator, vowing to destroy all Devils while others head back to the Abyss to tell of what they have found.

Now, each side has learned of the other and they begin amassing raiding parties to attack the other side. These parties soon grow into battalions and then swell into the size of armies and beyond. They begin slaughtering each other and it looks like no side will win against the other. The Demons have what feels like an infinite number in their armies, though they are too chaotic to make an effective fighting force. The Devils are more limited in their numbers, but due to their lawful nature can create effective armies capable of stopping the chaotic horde.

After centuries of conflict, the Yugoloths sent emissaries to both sides and offered their services as mercenaries, those most contracts benefitted them the most. Many point to this as the time that the Yugoloths began their plan of controlling the Blood War and in the first battle, they betrayed both sides and broke their contracts. Though this did nothing to stop either side from hiring them again and again after that, for when the Yugoloths held up their contracts they were the key to winning decisive battles.

Soon after, the Lords of the Nine appear in Baator and Demon Princes appear in the Abyss. Each plane suffers its own massive power struggles until the Lords and Princes put in a hierarchy that stops ambitious friends from unseating them. These leaders begin pushing their armies harder and the war continues to rage across Baator, Gehenna, the Gray Wastes, Carceri, and the Abyss with no side gaining the upper hand for very long.

The War Spreads Out

At this point, both sides realize they are in a stalemate and they decide to send out scouts to see what else is out there and if there is anything that can be used in the Blood War. This decision brings with it the discovery of the Multiverse and each side realizes that if they simply destroy the other side and end this conflict, they could have all the territory of the Multiverse and the battles are fought with an intensity beyond what was previously seen.

While the fiends are exploring the Multiverse, they attract the attention of the Celestials who abhor their evil and a massive army of Angels, Devas, Solars, and others are sent to the lower planes and begin massacring any fiend that gets in their way. The Blood War is now the fiends against each other and the Celestials against both sides. After a full year, the fiends realize they must work together to stop the Celestial onslaught and they turn their attention to the Celestial host. In less than a week, the Celestials are driven out of the lower planes with their white robes stained in the blood of their fallen kin. All told, it is said that of the massive host of Celestials that defies numbering, only 3000 are to survive this assault. The Celestials that wish to remain a part of the Blood War realize they must pick a side, but none can agree if they should join the Devils and Law or the Demons and Chaos as Celestials lay claim to both ideologies.