Chaos offers power in exchange for worship - a chance for even the weakest link of a society to rise above their peers into a position of domination and, eventually, immortality. It is like the dark side of the Force - easier, faster, more tempting. All you have to do is dedicate all your efforts to one of the four Ruinous Powers - or all of them, if you believe in that sort of thing - and your rewards will take your physical and mental abilities beyond those of a baseline human (or whatever species you happen to be part of).



Slaanesh promises sensual bliss, Nurgle grants durability, Khorne's gift is martial strength and Tzeentch is the giver of knowledge and understanding.



All they ask for in return is blood: yours or your enemy's.



Once you sell your soul to Chaos, there are only two directions you can go: daemonhood or damnation.

Your successes will be rewarded - as you conquer and kill in the name of your god(s), you reach new sensual heights, you become more resistant to damage, you are faster and stronger in combat, or your mind opens up to deeper knowledge of magic and even time itself. All of this very likely reshapes your body to reflect your new abilities and the traits of the entity you serve and you turn into a hideous monster by any human standards, but you don't care, for it is the will of your deity and thus it is holy.

Everything also has a flipside: your body dulls to previous excesses and forces you to seek greater and greater sensations through any means necessary; your damage resistance leads to your physical form turning into a pustulent sack of rotten meat that really can't fall apart any further; the thrill of battle sends you into an insatiable and blind bloodlust that drives you to kill, kill and kill, not caring if it's friend or enemy that gets in the way of your axe, because your god doesn't care either; and those who deal with the Architect of Fate will soon understand more than they can possibly know and tread close to the danger of losing all sense of time, place and self as all the timelines and of plots of the cosmos flow through their minds, and at the same time they're beset from all sides by the conspiring of daemons, rival sorcerers and the Changer of Ways himself.



Sacrifice enough in the name of your lord and you will be rewarded with ascension to an immortal aetheric form. This very likely will cost you your life, your friends, everyone you (could have) cared about, your world, your people and eventually your body, but you don't care, for what awaits you is infinite excess, righteous pestilence, glorious slaughter or all-encompassing wisdom and magical prowess, and most importantly undeath in the dark glory of Chaos.

Failure, however, is not tolerated: even a moment of shame for your dark patron may result in all your achievements being swept away, you losing your position to your second-in-command and finally your body deforming into a mindless mutant horror that can't sustain itself for long and quickly dies, after which your soul will be cast adrift in the Warp to be torn apart by the claws of impossible creatures.



It really is the ultimate gamble of all-or-nothing and it very effectively plays to the egotism festering inside all humans - every worshiper, no matter how lowly or mighty, believes themselves to be the chosen one, the righteous heir to the power of the Warp, and they will rape, contaminate, murder and conspire to ensure their place in the ivory tower. This is why certain people (criminals, traitors, deserters, disillusioned zealots etc.) are drawn to Chaos - they have nothing left to lose, and the path they step on can only lead to immortality or the fires of Hell itself, and they're well on their way towards the latter anyway, so to them it's well worth the risk.



It really is Lorgar's fault, all of it. Chaos is an old religion, older than the Imperium, but the primarch of the Word Bearers was the one to write the literal book on the philosophy - which was technically correct - that humanity is the chosen species of the Dark Gods and could be elevated to unprecedented daemonic glory in the eldritch halls of the Realm of Chaos. He saw the Gal Vorbak, his Blessed Sons and the first symbiotic (possessed) fusions of Neverborn and Astartes, as the next step in humanity's evolution, being both daemonic and physical, existing in two realities simultaneously, sharing two souls in one body, and this ideal has spread throughout the entire faith of Chaos Undivided and the separate cults of the four gods - that only through servitude to Chaos can one transcend mortal humanity into something revered, loved and feared. It tempts the weak, because the weak want to be strong, and it draws the strong for their desire to be stronger, indomitable. What most of them are never told is that with great power comes great insanity, and only those clever enough to strictly keep their mind on the right track through the excess, the decay, the slaughter and the madness can reach the final reward. Everyone else will turn into dust beneath the rising Chaos lord's boots.

But ultimately all of this really is of no concern to the Primordial Annihilator - as one dark champion ascends, millions or even billions of souls burn on the wayside, and Chaos will always have blood: yours or your enemy's.



Long answer, but I just couldn't stop writing.