Unaware of the rise and spread of the Heresy, but somewhat perplexed by Horus's orders, the Primarch of the Ultramarines Legion Roboute Guilliman returns to Ultramar with the intention of mobilizing his warriors to repel the massive Ork invasion of the Veridian system. Meanwhile, the Ultramarines' supposed ally in this battle, the Word Bearers Legion, staged an invasion of Calth, the second most important world of the Ultramar system, scattering the defenders' fleet and slaughtering all who stood in their way. This is the worst scenario Guilliman could imagine. This means that his brother Lorgar decided to put an end to their fierce rivalry in such a crazy way. But it is only when the traitors summon their daemonic masters and the Ultramarines are attacked by the hordes of Chaos that Roboute realizes just how far his brother's madness has gone. And he also understands that in this fight neither side can win...