Book three in the Space Marine Battles series.

A warleader of the renegade Alpha Legion, the demon prince Kernax Voldrius's reign of terror has bled across the stars, leaving billions dead. Captain Kor'sarroKhan of the White Scars Chapter is petitioned to hunt down and destroy Voldorius. Tracking the beast  doggedly for over a decade, Kor'sarro Khan finally drags Voldorius to battle on Quintus, a world that has totally fallen to the Alpha Legion. Now together with their Raven Guard allies, the White Scars must combat an entire planet if they are to slay the daemon price.