Essence Thief is a medium armor set. When you deal damage with a Light Attack, you draw essence from the enemy. Collecting the essence heals you 3900 Health, restores 3900 Stamina, and increases your damage by 10% for 10 seconds. Essence lasts 5 seconds, and spawns close to your target.
The Essence Thief set is part of the Imperial City DLC. It drops in White-Gold Tower in the Imperial City. Each set item is bound on pickup. You can acquire medium armor, weapons and jewelry of this set in the Xivkyn motif style.
If you need more information about the Imperial City, make sure to check out the full Imperial City Guide