Call of the Devourer

This item is used to craft a Bone Effigy used to summon The Many-Faced Devourer. Carrying it will allow the acquisition of the following needed bones: Ur'zul Bone, Imp Bone, Fiend Bone.

Source
This item drops from the following demons in the Scavenger's Boneyard in the Antoran Wastes:
 * Antoran Defender
 * Tormented Ritualist

As a quest objective

 * Immortal Ace