Dark Heresy: An Acolyte on the Roof
The Red Bishop
Former Grand Cleric of Hive Kellia, fallen from grace and has embraced dire sorceries.
Encountered in Hive Kellia during the Kellian Succession, it was found this horrid heretic orchestrated the whole thing. Found to be a sorcerer of great power, the Acolytes harmed him leading him to flee.