Negative Energy

Negative energy is the force behind the undead, the reason corpses rise and the reason the undead go on existing among the living. Undead are born out of the negative energy gathered in an area and the stronger ones take a great deal of it: crafting a skeletal dragon calls for two months of elaborate ritual and an immense quantity. It is understood as the resentment of the dead, which is why prayers and careful burial are used to settle it. In practice it works as the mirror of the energy of the living. An elder lich can inflict it by touch, and armor is no protection, since it passes straight through and drains the vitality of anyone living. The spell [Protection Energy - Negative] guards against that. It heals the undead instead of harming them, so attack spells built on it do nothing to an undead foe.