A forge world is a planet given over entirely to industry and manufacture, ruled not by the Imperium's usual governors but by the Adeptus Mechanicus, the tech-priesthood that safeguards and worships technology. These worlds are the great workshops of humanity, producing the weapons, vehicles, munitions, and machines without which the Imperium's endless wars could not be fought.
A typical forge world is a blasted, smog-wreathed sphere, its surface buried beneath sprawling manufactorums, refineries, and towering chimneys that blot out the sky. The tech-priests who dwell there venerate the Machine God, or Omnissiah, and treat their labor as an act of devotion, tending to production lines and ancient technological relics with equal reverence. Many host vast forge-cities and are defended by the Mechanicus's own armored legions and battle-servitors.
The most powerful forge worlds hold the sacred patterns for constructing the most complex machinery, and their output can dictate the fortunes of entire crusades. Their loyalty is to the Machine Cult first and the wider Imperium second, an alliance formalized in ancient compacts. Rich in industry yet often barren of natural life, a forge world is the beating industrial heart of the Imperium, where the smoke of production never clears.