From back cover:
Long years have passed since the Soul Drinkers Space Marines were declared ex-communicate and many Imperial agents have been despatched to destroy this once loyal Chapter. Sarpedon, the stoic leader of the fallen Marines, has become hellbent on finding some way of curing his battle-brothers of the cursed mutations inflicted upon them. After numerous false trails and dead ends, he has stumbled upon the most tentative of leads which promises them a final redemption in the eyes of the God-Emperor. But can they stay alive long enough to prove their loyalty?