The age of the Dark Imperium has begun, and the human race is poised on the brink of ruin.

In their darkest hour, the Emperor's servants have achieved the impossible: the resurrection of the Primarch Roboute Guilliman. Now Lord Commander of the Imperium of Man, Guilliman marshals his forces in a desperate effort to drive back the predations of Chaos: the Indomitus Crusade. Dispatched ahead of the bulk of Guilliman's war fleets, Chaplain Helios of the Ultramarines is entrusted by the risen primarch with a mission of vital importance. Will he achieve a crucial victory for Guilliman in time, or will a millennia-old obsession spell his doom? And just what manner of weapons will be needed to wage the war to save mankind?