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Upon the breakdown of the Old Ones' warp gates, Chaos entered the world in the form of warpstone dust which rained down upon the world, warping many species into monstrous new forms, as well as leaving a far more subtle and insidious mark upon other races.
Warpstone of more substantial size can also be found throughout the Warhammer world, but especially in the Chaos Wastes and where comets or meteors have fallen to the earth. Warpstone usually takes the form of an irregularly-shaped lump about the size of a man's fist.
In its solid, unrefined form, warpstone seems to swallow all light around it, creating a small patch of darkness around it. It also radiates a deadly and corrupting energy; creatures suffering prolonged exposure eventually die or become warped by mutation, although this may not affect the Skaven, as many of them wield weapons and talismans containing unrefined warpstone with no obvious dangerous and corrupting effects (although it may explain their backstabbing and egotistical nature).
Warpstone can be refined by Skaven Grey Seers into a grey powder, allowing it to be used in more controlled ways. While the Skaven use and rely on warpstone, the use and possession of warpstone is strictly forbidden in any civilized human society like The Empire. Nevertheless after the destruction of the city of Mordheim by a comet of warpstone, adventurers throughout the Old World flocked to the ruins in hopes of finding the dangerous but precious fragments of the comet.
Throughout history, comets of warpstone have fallen to the earth, the largest often creating vast craters suffused with corrupting magical energy. The best known of these include:
- the ruined city of Mordheim
- the ancient city of Kavzar - now known as Skavenblight
- the Warpstone Desolation around the Great Maw east to the Mountains of Mourn
- the cursed land of Sylvania, where lots of smaller warpstone meteorites rained down during the Great Plague
- Cripple Peak where Nagash built his fortress Nagashizzar. Nagash referred to the substance by the name abn-i-khat, which translates to 'burning stone' in Nehekharan
- A Warhammer Fantasy Roleplay fan magazine is also named "Warpstone".