Necrotic

A necrotic weapon deals cold damage infused with negative energy. Creatures immune to negative energy (such as the targets of a death ward spell) are immune to the cold damage of a necrotic weapon, and the cold damage of necrotic weapons affects only living creatures. Undead creatures targeted by a weapon with this property not only take no damage from the cold but also gain temporary Hit Points equal to the weapon’s item level. These temporary Hit Points last for 10 minutes, until expended, or until the undead gains a larger number of temporary Hit Points from a necrotic weapon. A creature can benefit from only one source of temporary Hit Points from a necrotic weapon at a time.