Goge Vandire
Jackass who took control of the imperium by taking up the positions of Ecclesiarch and Head of the Administration at the same time, starting the Age of Apostasy.
Murdered so many innocent Imperial citizens in a fit of paranoia that even the Imperium (the Catholic Nazis of the setting) views him the same way non-Nazi people in real life view the actual and universally despised leader of the Nazis.
Took a shitload of spes muhreens to take on the faggot, and even then he only died because his female bodyguards finally realized he was an asshole and cut off his head, possibly by order of The Emperor himself. At least he left behind some epic last words of "I don't have time to die; I'm too busy" which probably was adapted into some nice comedy movies for the Imperial citizenry (those that can afford to watch movies or study history in the first place, at least). A good example of how no matter how bad the Imperium gets, it could always be (and has been) worse. What a cunt. But don't worry because the imperium won't get any worse because Gee Dubs won't advance the story.
The Plan
Even before taking power, Vandire got his way to becoming Master of the Administratum by back-stabbing, bribery and assassination.
When the current Ecclesiarch died, He managed to wrangle his own choice of Ecclesiarch into the grimdark Vatican, who ended up being the weakest guy to ever serve in the role. Thus when confidence in the church started to plummet he actually had a case for storming the palace, where apparently the space-pope was involved in some sort of debauchery that would put even the Borgias to shame. So he pronounced the Ecclesiarch guilty and took the job himself - what a hero
Age of Apostasy
Absolute power corrupts absolutely, especially when the guy claiming the power was corrupt already.
Vandire went insane (if he wasn't already) with power, and started claiming that he spoke with the voice of the Emperor himself, basically a "whatever I say, goes" and started getting clerks to write down whatever he said at all times.
He then started issuing crazy edicts