Demon Skin

Demon Skin was a Legendary Crafting Material in Diablo III. It requires a Character Level of 1 to use, is account bound, and may be stacked up to 1000 times. It was required for the crafting of each piece of the Demon's Hide set. As of patch 2.0.6., it no longer drops, and existing Skins may be sold for 50,000 Gold.

The item had a chance to drop off the unique Phasebeasts and Armored Destroyers:
 * Axgore the Cleaver (Arreat Crater)
 * Haxxor (Tower of the Damned Level 1)
 * Brimstone (Tower of the Damned Level 2)
 * Grimmaw (Tower of the Damned Level 2, bugged)
 * Khatun (Gardens of Hope Tier 1)
 * Razorclaw (Gardens of Hope Tier 2, "The Light of Hope" quest)

The skin seems to phase out of existence and then return again as if nothing happened.