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.

Inside this 240-page hardback book, you'll find:

- The Years of Darkness and Blood – An in-depth exploration of the bitter and futile wars that raged across the galaxy in the last days of the Horus Heresy, including a detailed timeline, overviews of all Loyalist and Traitor forces, the rise of the Legiones Inductii, and the full narrative of the world-shaking Siege of Cthonia.

- Campaigns of The Age of Darkness: The Siege of Cthonia – A full Onslaught Campaign system, providing rules for a linked series of games between 2-6 players, where each victory or defeat builds up to a final battle. Also included are specific rules to tailor your Onslaught Campaign to the Siege of Cthonia.

- Age of Darkness: Core Missions – Five new Core Missions, balanced for both campaign games and Matched Play events. Race to claim vital strongpoints in Sudden Strike, adapt to shifting tides in Strike and Fade, or battle for the central ground in The Heart of Battle.

- Siege of Cthonia: Apex Missions – Two new Apex Missions, with extensive special rules intended for campaigns and narrative games. Fight through an ill-fated orbital assault of grav-mines and dropship hulks in The Pyre of Ambition, and survive an ambush from underground bunkers in The Ashes Of Victory.

- Age of Darkness: Zone Mortalis – Full rules for Zone Mortalis games, smaller-scale battles of Warhammer: The Horus Heresy fought within cramped war zones such as disputed spaceports, besieged hive cities, and winding fortress tunnels. Includes extensive rules for terrain and visibility, deployment and reserves, Reactions and Objectives, and army selection.

- Zone Mortalis: Core Missions – Three new Core Missions and two new Apex Missions for Zone Mortalis games, plus specific rules for Cthonian Underworld Zone Mortalis terrain such as Subterranean Tunnels and Promethium Condenser Tanks.

- Legiones Astartes: Rules Addenda – Extensive additional rules, units, and upgrades for use by the Sons of Horus, Imperial Fists, and all Legiones Astartes armies, including:
– Legiones Astartes: Imperial Fists – Wargear and Warlord Traits for Imperial Fists armies, and a profile for Lord-castellan Evander Garrius, the Tyrant of Cthonia.
– Legiones Astartes: Sons of Horus – Wargear and Warlord Traits for Sons of Horus armies, and a profile for Vheren Ashurhaddon, the First Reaver.
– Legiones Astartes: Legiones Decurion – Legiones Decurion rules for skilled tank commanders, including two Decurions for all Legions and one each for the Imperial Fists and Sons of Horus.
– Agents of the Cults Abominatio – Rules for fielding Cults Abominatio Operatives in Traitor armies, including a profile for the Infernus Abomination.
– Legiones Astartes: Inductii – Rules for fielding Inductii, the hastily-created Astartes used as reinforcements in the latter days of the Horus Heresy, with specific rules for each Legion.

- Showcase – Along with atmospheric photography and annotated artwork, the book includes a showcase of beautifully painted miniatures for the Sons of Horus, Imperial Fists, Traitor Allies, and Loyalist Allies.

In order to use the contents of this book, you will require the Warhammer: The Horus Heresy – The Age of Darkness Rulebook and one of the Army Books for Warhammer: The Horus Heresy.