Skull

A Skull provides Attacker Takes Damage of 12 when inserted in shields or off-hands, Replenish Life +3, Regenerate Mana 12% in body armor and headgear, and 2% Mana stolen per hit, 3% Life stolen per hit in weapons.

A Skull requires level 12 to use. When you socket an item with a Skull, you increase the level requirement to level 12, unless it was already higher.