Mars is the fourth planet of the Sol System and the most sacred world of the Adeptus Mechanicus, the ancient priesthood of the Machine God that oversees the Imperium's industry and hoards its technological lore. It is the first and greatest of all the Imperium's forge worlds, its ochre surface long ago buried beneath a global sprawl of manufactorums, refineries, and towering hive-forges whose furnaces have not cooled in ten thousand years.
Once the site of humanity's earliest ventures beyond Terra, Mars gave rise to a civilization of tech-adepts who came to worship the machine as an expression of the divine. Though bound to the Emperor by ancient treaty, the Red Planet retains its own culture, its own armies of skitarii and battle-servitors, and its own vast repositories of forgotten knowledge. Its rulers, the Fabricator-General and the high magi of the Priesthood, wield influence that rivals that of Terra itself.
Beneath the smog-choked skies lie labyrinthine vaults where the secrets of the Dark Age of Technology are hoarded and slowly, imperfectly understood. Mars exports war machines, weapons, and munitions to a thousand battlefronts, and no crusade of consequence sails without its blessing. To gaze upon the red planet is to see the true engine of the Imperium, a world where faith and iron are forged into one, the archetype from which every other forge world descends.