Fabius Bile

Told you.

Unlike you, whelp, I once walked the same ground as your Idol. I breathed the same air as him. And I tell you this, without lie or artifice. He never wanted to become what you have made him! He did not wish to be your god-thing. He abhorred such ideals! The slavery of your crippled, blind Imperium would sicken him, if he had eyes to see it. — Fabius Bile, telling it like it is to Rafen of the Blood Angels in Black Tide.

Fabius Bile (Fabulous Bill) is the mad scientist to end mad science (since to the Haemonculi science comes second to pain). 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.

Send in the Clones

Bile has a hard-on for cloning technology, and has a track record for making copies of himself and others.

His first attempt was actually to clone Ferrus Manus since Fulgrim was feeling a bit guilty and wanted a do-over on turning his brother to chaos, though Fulgrim would keep murdering the clones because they wouldn't take the hint and change sides.

He later 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. (And as it turns out, they didn't kill him- he had actually made several clones of himself to be used as body doubles.) Also responsible for creating Honsou. Sort of. Jury's out as to whether this is a good thing.

Cancer?

There is a short story about Fabius' origins in Blades of the Traitor, where he's narrating through the early years of the III Legion. He was the Apothecary they tasked to figure out a cure for the flaw in the gene seed that was causing the Emperor's Children to die out and number so few in the early days of the Great Crusade. It turns out that a cure was never actually found, and that reuniting with their Primarch on Chemos only served to provide fresh material for "untainted" space marines.

All those who suffered the flaw were put to death in an attempt to prevent any infection from reaching the healthy brethren... Except for Fabius himself who was in a position to falsify his medical records. The story reveals that he suffers the same condition as his tainted brothers, which is basically a form of cancer that causes tumours all over their internal organs and that Fabius is probably the last sufferer remaining, though he is still searching for a cure.

His prognosis is that he only has about a year to live, but is in the process of temporarily rejuvenating himself using elixers distilled from enzymes and proteins drawn from dead Astartes, and quite possibly including those who never even suffered the flaw either. (Only hinted at early on, though we can fully expect this from him later) This is likely part of his motivation to start modifying the Emperor's Children astartes after conquering Laeran.

Its currently uncertain how he managed to survive into the 41st millennium, though the story ends with his decision to confess his condition to Daemon-Fulgrim, so he's either found a cure / is extending his life through more unnatural means / is cloning himself over and over with each clone having less than a year to live