Having declared its secession from Imperial rule at the very dawn of the Horus Heresy, Mars sat like a viper upon the doorstep of the Loyalist Throneworld – a locus of industry which toiled ceaselessly in preparation for the inevitable Traitor assault upon Terra. Loyalist incursions throughout the civil war would attempt to stymie these efforts, but eventually, the power of Mars would be unleashed against the Sol System in a maelstrom of violence. This expansion book for Warhammer: The Horus Heresy digs deep into the conflict on Mars, home to some of the most bitter fighting in the Age of Darkness. Recreate the key battles with thematic Relic Hunt missions, a new campaign, and rules for additional units. Inside this 168-page hardback book, you’ll find: In-depth Lore and Background: Timelines, maps, and more covering the cataclysmic events on Mars during the Age of Darkness. From the Schism of Mars, the birth of the Mechanicum, and the key conflicts involved in the the Martian Civil War. The Mars Campaign: The Onslaught Campaign System allows you to play linked battles of Warhammer: The Horus Heresy. Includes 10 x core campaign stratagems, optional rules, and the Mars Civil War Campaign. Core Missions: Provides 5 x Core Missions: The Crucible of War; The Heart of Battle; Sudden Strike; Take and Hold; and Strike and Fade. Apex Missions: Provides 3 x Apex Missions which can be played as a linked set of campaign games using the Onslaught Campaign rules, or as one-off missions. Dark Scions of the Questoris: Includes options for the Divisio Tactica: Questoris Household list found in Liber Mechancium. Automata Malefica: Rules for using Corrupted Engines within a Traitor Mechanicum force. Relic Hunt Missions: 5 x Relic Hunt Missions: Data Coil Download; Unlock the Vault; Secure the Crash Site; Relic Extraction; and Convoy Raid. Techno-heresies of the Dark Age: Rules for Proscribed Techno-arcana, accessible by some Traitor forces: Malefikite; Machin Malefica; Ruinous Cybernetica; Nekrotek; Ephemera Nekrotek; Malamina; Æthertek; and Anima Malefica. Additional Unit Profiles: Includes 3 x Legiones Astartes Army List profiles and 1 x Mechancium Army List profile for the Krios Squadron. Miniatures Showcase: A full-colour showcase featuring various Mechanicum miniatures. 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, which are all available separately.