The galaxy is in flames. Chaos is in the ascendant across the stars. The Great Rift has split the holdings of the Imperium in twain, isolating entire sectors from the light of Holy Terra.
But all hope is not lost. The Primarch Roboute Guilliman has returned from deathless sleep, and appointed Commander Dante, lord of the Blood Angels, as Regent and Warden of the newly dubbed Imperium Nihilus.
In the Baal system, the shattered holding of the Chapter is being rebuilt, and Dante plans the greatest campaign of his long life, to retake half an empire. And yet at this moment of rebirth there are dangers close to home that could overwhelm all those who carry the blood of Sanguinius in their veins, stopping Dante's noble endeavour before it is begun.
The Flaw in Sanguinius's sons is growing. As the twin curses of the Red Thirst and the Black Rage threaten everything, the hardest ordeal will fall upon Mephiston, twice-born Lord of Death and Chief Librarian of the Blood Angels. Among the mighty lords of Baal, perhaps only he can save them all, by mastering the darkness in the blood...