Tainted Essence

Source
The essence are dropped by members of the Cult of the Damned:
 * Cult Taskmaster
 * Void Summoner
 * Cult Alchemist
 * Cult Blackguard
 * Damned Apothecary
 * Shadow Channeler
 * Vile Torturer
 * Dark Subjugator
 * Cult Researcher
 * Cultist Shard Watcher