The Horus Heresy is civil war on a galactic scale, a conflict comprising countless bitter campaigns and doomed struggles. In the latter years of the Age of Darkness, the Loyalist forces occupying the Sons of Horus home world of Cthonia face a determined assault from overwhelming Traitor forces, who seek to oust them from the ancestral home of the Warmaster Horus. The resulting war of attrition rages in the skies above and the tunnels below the surface of that embattled world, taking a terrible toll on Loyalist and Traitor alike.

This expansion book for Warhammer: The Horus Heresy provides a comprehensive history of the Siege of Cthonia, detailing the Loyalist and Traitor forces involved in the campaign. It also allows you to put your army in the role of Loyalist occupiers or Traitor resistance, with rules for linking your games into Onslaught Campaigns, playing new Core Missions and narrative Apex Missions, fighting Zone Mortalis battles in cramped conditions, and fielding new units such as unstable Legiones Inductii recruits, Legiones Decurion tank commanders, the hellish Infernus Abominations, and upgrades and unique characters specifically for the Imperial Fists and Sons of Horus.