Great swathes of Mars seceded from Imperial rule during the Horus Heresy, and the tireless deluge of machines and weapons pouring from its factories posed a mounting threat to the throneworld as the Warmaster drew ever closer. Loyalist forces attempted to wrest control of the planet on numerous occasions, even as the Tech-priests squabbled among themselves, and these events provide the backdrop for the next supplement for Warhammer: The Horus Heresy, Campaigns of The Age of Darkness: The Martian Civil War.