Warpstone is a highly toxic stone-like material, said to be made from the condensed magical energies of pure Chaos that soldify in the Mortal Realms.
Skaven make use of it heavily, using it to power their technology, as a poison and to create mutants like Stormfiends.
Similarly, it is said that unprotected exposure to warpstone can turn a human into a skaven and, for both aelfs and duardin, can (and does) lead to insanity.