The Ultramarines are the epitome of a Space Marine Chapter. Warriors without peer, their name is a byword for discipline and honour, and their heroic deeds are legendary. The Great Devourer is descending on the world of Tarsis Ultra, a world under the protection of the Ultramarines. Captain Uriel Ventris and his warriors - allied with their barbaric cousins from the Mortifactors Chapter - rally the forces of the planet to defend against the tyranids. But to defeat the xenos horrors, Uriel may have to turn his back on the Ultramarines' teachings and embark upon a dangerous and unorthodox mission.This edition also includes the prequel short story 'Leviathan', which sees Ventris and his Ultramarines board a space hulk an encounter the vanguard of the hive fleets: the feared genestealers.