Essence of Undeath

Essence of Undeath is an elemental building block of many crafting recipes. It adds the virtues of death and darkness to any item.

Source
Essence of Undeath drops off of Undead mobs in Dire Maul, Eastern Plaguelands, Naxxramas, Scholomance, Stratholme, Western Plaguelands, and Zul'Gurub.

This item can also be transmuted using Alchemy (275); taught by Recipe: Transmute Water to Undeath.

Alchemy

 * Transmute: Undeath To Water

Blacksmithing

 * Corruption (sword)
 * Annihilator
 * Persuader

Cooking

 * Haunted Herring (Essence of Undeath is the only item used as a cooking ingredient)

Enchanting

 * Enchant Weapon - Unholy Weapon
 * Shadow Power
 * Lifestealing

Engineering

 * Gnomish Death Ray
 * Ultra-Flash Shadow Reflector

Tailoring

 * Robe of Winter Night
 * Felcloth Gloves
 * Robe of the Void

Jewelcrafting

 * Sapphire Pendant of Winter Night
 * Ring of Bitter Shadows