Denied but not defeated, the Traitors intensify their assault on the Imperial Palace. With the principal space ports in Horus’ hands, the Warmaster drains the heavens of his reserves as the power of Chaos waxes. As the fabric of the defences and the will of those who hold them cracks, Horus commands the Titans of the Legio Mortis to breach the walls. Against them stand the might of Mercury Wall and the strength of the Legio Ignatum. Ancient rivals, the god-engines of both Legions meet in battle.