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. He doesn't follow any Chaos God despite being aligned to Chaos (he used to be in the Emperor's Children, but left before they became famous they ran him out because he wouldn't worship Slaanesh exclusively) and is really only interested in creating the most fucked-up creatures imaginable. 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, 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. 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.