Bloodthirst is a regular enhancement that can be found in the Twilight Cathedral. In the room down an elevator from the room where War fights The Jailer, War must use the Crossblade to carry fire from torch to torch until every torch in the room is lit. A chest with Bloodthirst inside then appears. When equipped it gives your weapon a green glow. Bloodthirst has the following bonuses:
Causes a portion of War's weapon damage to be converted into health for him.
The health gain from slotting this enhancement is very minimal. You will need to kill multiple enemies to regain the health lost from a single attack made against you. It is most useful when going back through areas you have already cleared out that don't have green health chests anymore.