The super-chaplain of the Sons of Prism (an Emperor's Champion on the tabletop). Due to the desperate state of the chapter, otherwise-dubious-or-unsavory actions are allowed (such as the chief librarian being a mutant), so long as they remain spiritually within the will of the Emperor.
When the weight of their situation becomes too much for his battle-brothers to bear, Xipe Totec, the Flayer, is called in to execute those who would turn traitor and jeopardize the Sons of Prism.

On the tabletop, the Champion's sword is AP2, and can be wielded either two-handed for +2 Strength, or inflict Instant Death, so I decided to give him the biggest, meanest, sword I could possibly find.