Naglering

The Naglering is a Level 51 quality Ring that improves Stamina. On equip, it has a chance to deal significant Arcane damage to any target that strikes it's wearer.

Source
This item drops from Golem Lord Argelmach in Blackrock Depths.

Trivia
Naglering pays tribute to Nagelring in Diablo II, a unique ring known for its large boost to magic find. They both share the bizarre feature of having the attacker take damage.