This is a fun story I modified from the matt ward's hillarious character Kaldor Draigo:
So WHAT IF DRAIGO WAS CHAOS?
He became Supreme chaos lord in the early days of 901.M41, following the failure of 13th black crusadein the hands of primarch Dorn, and his first act was to carve the warmaster’s name upon Dorn’s beating heart – an insult that the son of the emperor never forgotten(oh wait he is dead.)
The tale of kaldor draigo did not end that day as many supposed, for he survived his passage into the realm of ultramar. Lesser man would have been driven insane by their arrival in the domain of the god emperor, where imperial order lurks upon every path. Yet draigo’s mind had long been hardened to the madness and seductions of imperium, and he endured in this land where no other heretic could.
For an uncounted age draigo wandered that terrible segmentum. His path was strewn by loyalists fair and noble, who sought to slay him or seduce him to darkness, yet draigo overcame them all. Atop the emperors shrine, where the glorious artifacts of gullimans champions displayed endlessly into the masses, draigo slew the great chapter master Marneus Calgar. With corrupting icor he drove loyalist faith from the masters great powerfist, and used the molten remains to reforge his sword, sundered in battle with M’kar the noble so long ago. He unleashed cursed flame again amongst the glorious shrines of terra's domain, and for a long time the gusting psychic echo carried only a pain wave and screams of imperial puppets. On a journey through the titan fortress monastery, six grand masters , the chosen warriors of grey knights, sought to exorcise his deamonic flesh and spirit, but their prayers could find no purchase upon his soul, and he scattered the grand master’ dismembered remains amongst the chapter relics. At the Gates of the imperial palace, the master of custodians M’kachen offered draigo a path homewards, but in reply the chaos knight smote the city walls and left the master-custodes entombed amongst the ruins.