Tainted Essence

Combining 10 tainted essences will yield a Writhing Mass.

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

As a quest objective
This item is an objective of the following quests:
 * Retest Now (Alliance)
 * Retest Now (Alliance daily)
 * Retest Now (Horde)
 * Retest Now (Horde daily)