Warpstone

Warpstone is a toxic material, said to be made from condensed chaos. 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, in concerns to both Aelfs and Duardin, can (and does) lead to insanity.