Having declared its secession from Imperial dominion at the dawn of the Horus Heresy, Mars is preparing for conflict on the Loyalist Throneworld, a location that is being intensively fortified against the Traitor's assaults on Terra.

This expansion book for Warhammer: The Horus Heresy explores the conflict on Mars with thematic missions, new campaigns, and rules for additional units. Within 168 hardcover pages, it includes:

• Knowledge and Background: Timelines and maps covering events on Mars during the Age of Darkness.
• The Mars Campaign: The Onslaught Campaign System allows for the conduct of linked battles.
• Core Missions: Provides 5 core missions: The Crucible of War, The Heart of Battle, Sudden Strike, Take and Hold, Strike and Fade.
• Apex Missions: 3 Apex Missions that can be played as part of a campaign or as standalone missions.
• Dark Scions of the Questoris: Options for the Divisio Tactica list: Questoris Household.
• Automata Malefica: Rules for using destroyed engines in a Traitor Mechanicum force.
• Relic Hunt Missions: 5 Relic Hunt Missions like Data Coil Download and Unlock the Vault.
• Techno-heresies of the Dark Age: Rules for Proscribed Techno-arcana from Traitor forces.
• Additional Unit Profiles: 3 profiles for the Legione Astartes Army List and 1 profile for the Mechancium Army List.
• Miniatures Showcase: A colored showcase featuring Mechanicum miniatures.

To use the content of this book, the Warhammer: The Horus Heresy – The Age of Darkness Rulebook and one of the Army Books for Warhammer: The Horus Heresy are required.