For too long the Emperor's Children have been denied their birthright. Their once-glorious Legion has been fractured into disparate warbands, condemned to ravage world after futile world in the pursuit of ambition and excess. But no longer.

Lord Fulgrim - ascended Primarch of the III Legion, the Perfect Son, the Emperor-in-Waiting - has issued a challenge to his scattered warriors: vanquish the Imperial world of Crucible, and bring him the head of the Black Templar charged with its defence.

But Fulgrim's plans go far beyond the conquest of a single planet, and what should have been a swift and bloody offensive turns into a gruelling siege. With success uncertain, a world in flames around them, and the Legion turning their weapons inwards as well as out, the would-be champions are forced to confront a bitter truth. What must they sacrifice for victory and the favour of their Primarch, and what will be left of them when the war is over?