Nightbringer (Ultramarines #1)

Newly promoted Ultramarines Captain Uriel Ventris is assigned to investigate Pavonis, an imperial planet plagued by civil disorder and renegade elder raiders. Nothing is as straightforward as it seems, and wheels are turning within wheels. Uriel and his allies are forced into a deadly race against time to destroy their shadowy enemy - or the whole planet must be sacrificed for the good of humanity.