Promethium is the Imperium's ubiquitous fuel and combustible chemical, a versatile substance that powers engines, fuels weapons, and lights the fires of industry across a million worlds. Broadly analogous to petroleum and other hydrocarbons, it is refined, transported, and hoarded on an enormous scale, for without it the tanks, aircraft, and machines of the Imperium would grind to a halt. Wars are fought over the worlds and refineries that produce it, and control of promethium reserves can decide the fate of entire campaigns.
In its most familiar military use, promethium is the burning fuel spat by flame weapons, igniting into a roaring gout of fire that engulfs the foe. It also propels vehicles, heats forges, and feeds the furnaces of the great industrial worlds. Highly flammable and dangerous to handle, it must be stored and transported with care, and a ruptured promethium tank can turn a vehicle into a blazing pyre.
The substance is central to the operations of the Astra Militarum, whose armoured columns thirst constantly for fuel, and to the Adeptus Mechanicus, who oversee its refinement and distribution. Like so much in the Imperium, promethium is treated with a measure of ritual reverence, blessed before use and rationed with grim discipline. It is the lifeblood of an interstellar war machine that never ceases to burn.