The fortress world of Regium has a proud history, For millennia, it has endured when others have fallen to ruin and damnation, standing defiant in defence of the Imperium of Man. But now it is shaken. Parasitic plagues run rampant, and Regium's citizens are haunted by dreams of horrific monsters feasting upon their flesh and marrow. Lieutenant Castamon of the Ultramarines recognises these signs of an impending Tyranid invasion all too well. Rallying his warriors, the battle-hardened veteran launches a strike force to slaughter the xenos in the outer system, before it can reach Regium. But as this new tendril of Hive Fleet Leviathan proves unpredictable and deadly, Castamon and the First Company must harness all their skill, might, and zeal, or face utter annihilation.