Hereticus

Across countless worlds, the Inquisition pursues a secret war against humankind’s foullest enemies – the alien, the unclean and the unholy. When a battle with an ancient foe turns deadly, Inquisitor Eisenhorn is forced to take terrible measures to save the lives of himself and his companions. But how much can any man deal with Chaos before turning into the very thing he is sworn to destroy?

Read it because
Eisenhorn seeks the truth behind a string of disappearances centred around a travelling carnival and a pictographer whose images are more than they seem. Inquisitor Eisenhorn has saved worlds and beaten bad guys galore, in short, he's a hero. But he's also done some dodgy things, and they're coming back to haunt him... His struggle to justify his actions (especially the daemon summoning) make this an incredibly compelling read and rounds off the trilogy perfectly.