Unaware of the origin and spread of Heresy, but somewhat puzzled by Horus' orders, the primar of the Ultramarine Legion, Robout Gilliman, returns to Ultramar with the intention of mobilizing his warriors to repel a massive Orc invasion of the Veridian system. At this time, an alleged ally of the Ultramarines in this battle, the Legion of the Word Bearers, organizes an invasion of Kalt, the second most important world of the Ultramar system, sweeping a fleet of defenders and exterminating anyone who stands in their way. This is the worst scenario Gilliman could have imagined. So his brother Lorgar decided to put an end to their fierce rivalry in such a crazy way. But it's only when traitors summon their demonic masters and the Ultramarines are attacked by hordes of Chaos that Robaut realizes how far his brother's madness has gone. He also understands that in this fight neither side will be able to win.