Fabius Bile

Told you.

Fabius Bile is the mad scientist to end mad science. He's got a lab coat made of human skin and a pimpin' staff. Back during the Great Crusade, Fabius was the Chief Apothecary of the Emperor's Children. Even before the Legion fell to Chaos, he was starting to perform experiments on trying to improve the gene-seed with Fulgrim's blessings. While the Emperor's Children became wholehearted devotees of Slaanesh, Fabius didn't turn into a hedonistic sense addict, but rather rejected the Chaos Gods despite being aligned to Chaos. Eventually, the rest of the Emperor's Children drove Fabius out of the Legion, but by that point his skill in cloning meant that he had plenty of opportunities to gain new subjects for experimentation while providing clones for the remaining Chaos Space Marines to make recruits from. May have had something to do with the Obliterator virus (aka the turn-your-body-into-a-living-cannon virus) and there are rumors that he had a hand in screwing up the 21st Founding.

He is also absolutely fabulous.

He has the funny idea that he has anything to offer the Chaos Gods while they have nothing to offer him, who just string him along for the lulz.

He has the ability to hook up with a squad and pump them full of his pimp cane juice, making them get on board the Rape Train and head right for the front cabin. His pimp cane also causes Instant Death, though it counts as a regular weapon so he's likely to survive against anything decent armor (very bad given that counting the CSM codex, six the armories out right are MEQ, with the Necrons also being close behind). Put him in with a squad of Berserkers or Possessed who scored themselves power weapons and stuff them in a Land Raider. It's time to rev the Fun Bus the fuck up and have yourselves a party.

He stole Horus's corpse and cloned it repeatedly, an act that pushed Abaddon out of his emo loser phase into becoming the scourge of the Imperium. Really though, Abaddon was just annoyed that Fabius has like four arms.

He's currently attempting to create a Clone of the EMPRAH. Using Sanguinius' blood. Let it never be said that Chaos is nothing if not ambitious. The Blood Angels managed to kill him and get the blood back, but he had already begun developing the clone at that point. (He had also made several clones of himself as body doubles just for the occasion, too.)

Also responsible for creating Honsou. Sort of. Jury's out as to whether this is a good thing.