Fabius Bile

Fabius Bile, also known as the Primogenitor, is a Chaos Space Marine Apothecary, originally of the Emperor's Children Traitor Legion, who has repeatedly attempted to use his knowledge of cloning and genetic engineering to create superhuman beings under his control in a quest to match and then exceed the Emperor of Mankind's original scientific achievement in creating the superhuman Astartes.

Background
He was once an Apothecary of the Emperor's Children Traitor Legion, but has since left the Legion, possibly under a cloud, as it is said that "the traitor betrayed a second time". Fabius, an agent of Chaos Undivided, dabbles in genetic engineering and helps the Traitor Legions recruit new members in return for their protection. During the Horus Heresy he modified the sensory systems of his fellow Emperor's Children and their connections to their pleasure centers creating the Noise Marines. During the Siege of Terra it is known that he performed genocidal experiments on the civilian population. After the Horus Heresy, he worked to unlock the secrets of creating Space Marines. After narrowly escaping the Space Marines of the Salamanders Chapter on the world of Arden, he now resides on an Eldar Crone World in the Eye of Terror where he continues his unethical experiments, creating mutated vat-grown super-soldiers he calls "New Men", and other foul things - animal/human hybrids - called "Modificates."

Cloning Horus
At some point, he managed to clone Horus, thinking that this would, among other things, carry him the favor of Abaddon the Despoiler. Instead, Abaddon destroyed the Horus replicae, proclaiming it a "blasphemy", and ordered the Black Legion to raze Bile's Laboratoria and destroy his experiments. It is fair to assume that he has not been in good terms with Abaddon; since that time, he is known to act as a "free agent", pursuing his own agenda. He has also succesfully managed to clone himself, with several of his replicaes active in various places, as he is said to have secret facilities in a dozen locations across the Milky Way Galaxy. His arrogance and twisted amoral intelligence is more than matched by his sadistic cruelty.

Bile on Baal
In an elaborate scheme, he managed to set foot on the Blood Angels homeworld of Baal disguised as a well-known Magos Biologis, ostensibly to help Apothecae Majoris Caecus of the Blood Angels in unsanctioned efforts to clone Space Marines. As part of his plan, he tricked Caecus into procuring a vial containing the blood of Sanguinius. Bile swiftly appropriated the vial and then fled the planet, after setting loose dozens of feral near-indestructible mutant clones to help him effect his escape.

The Emperor Project
He intends to use the highly pure blood of Sanguinius to help him in an audacious scheme: obtaining the complete genetic code of the Emperor of Mankind and creating a replicae - that Bile would control - with all of the Emperor's psychic powers and other attributes, and none of his humanity. For ages, he had been collecting body parts and genetic material from captured or killed Space Marines to further this project. Sanguinius's blood, carrying a purer strain of the Emperor's DNA, was a huge leap forward. His base for this project was in a facility on Dynikas V, a world in the Ghoul Stars region, that was overran by the remnants of a splinter Tyranid Hive Fleet. Bile, thanks to a captured Zoanthrope he "modified", was using certain pheromones produced by the Zoanthrope to confuse the tyranids and hide the existense of his facility from their senses.

Eventually, in a joint mission, a squad of Blood Angels under Battle-Sergeant Rafen and a squad of Flesh Tearers under Veteran-Sergeant Noxx managed to approach Dynikas V, infiltrate Bile's compound, and attack the laboratory. Unbeknown to them, Bile had four clones of himself in the facility. Rafen managed to kill all of them, the last as it was ready to teleport away, but only after Rafen temporarily attained enormous strength and power by injecting himself with the blood of Sanguinius that Bile had stolen. Bile however, had told Rafen that he did not need the blood anymore, as he had analyzed it already, and knew its secrets.

Other Info
Following these events, the presumably "real" Bile is shown alive and well on his Crone World. His knowledge of the Emperor's DNA is intact, but the replicae project has suffered major delays thanks to the Blood Angels and Flesh Tearers' actions. He is then offered the alliance of the Daemon Prince Malfallax in taking revenge against the Blood Angels, although it is not currently known whether he'll accept the offer.

Fabius experiments on himself via his Chirurgeon, an archaic device normally attached to his back and fueled by Warp-spawned fluids, resulting in vast variations on his strength and other abilities. On the battlefield, he might also bestow the questionable gift of his enhancements to others, with uncertain results. Sometimes the experiment is a success, creating psychotic killers superhuman even in comparison to their corrupted Chaos brethren, but sometimes the subject's metabolism disintegrates in the face of the stress, resulting in instant death. Bile also employs other obscure or Archeotech weaponry, such as the Xyclos Needler, a needle pistol that shoots toxin-laden darts capable of felling a Space Marine. Inspiration-wise, Fabius is something of an amalgamation of various "mad scientist" figures of both reality and fiction, from Josef Mengele to Victor Frankenstein.