Age of Sigmar Wiki

Warpstone is a toxic stone-like material, said to be made from the condensed magical energies of pure 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.