Unaware of how the Heresy began and spread, but somewhat puzzled by Horus’s orders, the Primarch of the Ultramarines Roboute Guilliman returns to Ultramar with the intention of mobilizing his warriors to repel the massive Ork invasion of the Veridian system. Meanwhile, a supposed ally of Ultramar in this battle—the Word Bearers Legion—is staging an invasion of Calth, the system’s second most important world, smashing the defenders’ fleet and slaughtering everyone who gets in their way. This is the worst scenario Guilliman could possibly imagine. So his brother Lorgar decided to put an end to their bitter rivalry in this mad way. But only when the traitors call their demonic masters and hordes of Chaos crash down on Ultramar does Roboute realize how far his brother’s madness has gone. And he also understands that in this struggle neither side will be able to win…