Warhammer 40k: Templar of Dust

The protagonist, Marshal Kael of the Black Templars, is given a mission by a member of the Ordo Malleus, a daemonhunter named Lady Ymira. He must join her in the destruction of a Chapter of Space Marines, the Knights Eternal, who have been declared Excommunicate Traitoris. She believes that the Chapter Master, Sven Baliik, has accepted the gifts of one of the Chaos Gods, and will soon be granted daemonhood. Kael chooses several Initiates and a Sword Brother from his men, and descends upon Requiem, the homeworld of the Knights Eternal. His goal is to purge the heretic, to kill one he used to call brother, but most of all: Kael is coming to destroy the unholy in the Emperor's name.