Basillius the Elder

Bastard Basillius the Elder was a Chaos worshipper who masqueraded as a loyalist preacher of the Ecclesiarchy, ascending to power over the Imperial Faith after the Ecclesiarchal Purges of 321.M37. In his guise as an Imperial Saint, he created an era in which the already minimal civil liberties granted to the Imperial citizenry were stripped away, resulting in huge slaughters as mandatory informing on suspected heretics and traitors was enforced throughout the galaxy. But this was only the first stage of his plan: when Warp Storm Dionys rippled into reality, hundreds of chapters of Space Marines whose homeworlds were within the path of the Warp Storm found themselves bedeviled by a wave of physical and psychological corruption, and "Saint" Basillius the Elder demanded they be rendered over to him for judgment. Such was his influence that within a year, they had been presented.

Several hundred were deemed to be free of Chaos' taint. Thirty instead failed his test. Deemed the Judged, they were voluntold for a redemptive crusade into the Eye of Terror (lest they be simply all executed). To the Eye they traveled, and into its depths they vanished in the cursed Abyssal Crusade.

Almost 800 years later, the last survivors of the Vorpal Swords Chapter fought their way free, angrily suffering tests of their purity before revealing what they had learned. Burning with righteous fury, they fought their way to Basillius' palace and publicly denounced him, revealing him as an agent of Chaos. The false saint and thousands of his followers perished, his shrines and remains were destroyed, and every relic and tome of his creed was burned, making him as much of an unperson as the Imperium can make someone.

And by "burned" we mean "loaded onto a freighter and launched at a sun."

Still, to this day, nobody knows just how far his corruption reached. Also, this is a good example of why you don't let a single guy tell a legion-sized force what to do, and before you point out, no, neither the First Founding chapter masters, not even Guilliman make the calls alone, even Bobby G works with a council who may point at him the obvious and has the Ten Thousand breathing down his neck in case he decides to take note from Horus' shenanigans.