Despite the destruction of Corax's plans to rebuild his Legion following the Landing Zone Massacre, the surviving Raven Guard remain unbroken. Coming upon a crippled Word Bearers ship far beyond the front lines, they discover evidence of a new alliance between the Traitors and the Forge World of Constanix 2, and learn of terrifying new engines of war stalking the battlefields across the sector. The threat is too significant to ignore. Primarch Corax must penetrate into the heart of the Mechanicum priesthood of Constanix if he is to gain allies for the coming war, although he may have to face the truth about his own mysterious nature...