Fabius was Chief Apothecary of the Emperor's Children before their reunion with Fulgrim. He was charged with trying to cure the legion's frequent mutations, but was increasingly unsuccessful with this task, and was forced to terminate many legionnaires. He too was alflicted but covered this up by fabricating his own records. This also lead to his experimental streak, as later in his career he began offering performance-enhancing body modifications.
Post-Heresy, Fulgrim was the one who gave Fabius his epithet of "Bile." He had since struck out on his own, with Fabius creating a clone from the stolen body of Horus as a show of his prowess. He also makes clones of Ferrus Manus for Fulgrim to try and convince him to join Chaos, though he always fails, and Fulgrim always blames on Fabius. One time he had been captured by the Dark Eldar, from which he was let go after he and the homonculi swapped notes on experimentation.
Fabius Bile agreed to allow Lucius the Eternal and Ahzek Ahriman to borrow his broadcast relay to air their Slaaneshmas special. He used the broadcast to announce that he was going to have a one-man Black Crusade, using just clones. To prove his point, he showed a horribly deformed clone of Ahriman, who promptly blew up the stage. He's also very triggered if people call him "Bob."