The Eisenhorn Trilogy Malleus

A century after his recovery of the alien Necroteuch, Gregor Eisenhorn is one of the Imperial Inquisition's most celebrated agents. But when a face from his past returns to haunt him, and he is implicated in a great tragedy that devastates the world of Thracian Primaris, Eisenhorn's universe crumbles around him. The daemon Cherubael is back, and seeks to bring the inquisitor to ruin - either by his death, or by turning him to the service of the Dark Gods.