Warpstone is solidified Chaos: the raw, corrupting essence of dark magic crystallised into a physical substance, glowing a sickly green and radiating a power that mutates, empowers, and destroys all it touches. It is the single most dangerous material in the world, and the most coveted by those willing to risk damnation for what it can do.
Much of it is said to fall from the corrupt moon Morrslieb, raining down as green meteors that salt the earth with corruption wherever they land. No creature covets it more than the Skaven, the ratmen of the under-empire, whose entire civilisation runs on warpstone: they eat it, forge weapons and war-machines fuelled by it, and wage endless wars in the dark to control its supply. Their warlock-engineers channel it into crackling lightning cannons and their grey seers gnaw it to fuel their sorcery, though it withers and warps them in the doing.
Warpstone is death to the unwary. To handle it is to court hideous mutation; to be near it is to sicken. Yet its temptation is bottomless, for a fragment can power spells beyond a wizard's natural reach, transmute base metal, or grant unnatural strength — at a price always paid in body and soul. Alchemists, cultists, and desperate men hunt it across the Old World, and many a village has been damned by a single green shard dug too eagerly from a meteor's crater. It is the physical proof that magic, unchecked, curdles into corruption.