Necrotic Powder

A grey powder carried in an alchemically treated pouch, Necrotic Powder magically ages anything it touches, whether living flesh or solid steel. Only practitioners of necromancy are immune to this effect. Usually the necromancer blows a handful of the substance at a target, which immediately inflicts terrible levels of entropy. Necrotic Powder may be used to rust metal, rot wood, or cause paper to decay into dust.

History
Developed by Necrarchs using methods first described by Nagash, Necrotic Powder is Dhar distilled into a physical form using Warpstone dust as a base. Those who carry Necrotic Powder have proven impossible to capture because a handful of the substance is enough to eat through any lock, as well as several guards.

Source

 * Warhammer Fantasy RPG 2nd ED -- Night's Dark Masters (pg. 123).