In space and on the worlds of Great Ultramar, the battle for the Imperium continues. Intent on rebuilding his homeland and using it as a base to rebuild humanity's devastated star empire, returning Primarch Roboute Guilliman continues his war to drive Mortarion and his Death Guard from the Ultramarines' domain. But when Guilliman finally confronts his brother on the disease-ridden plains of Parmenio, the interference of higher powers in their duel threatens to destroy the Imperial Regent's understanding of the world around him and his place in it.