Undead are creatures that do not live anymore, but instead are driven by negative energy. Since they don't depend on the functioning of organs, they are immune to sneak attacks, critical hits and usually poison and diseases. Negative energy damage usually heals them, positive energy injures them. Several undead, such as mummies, are vulnerable to fire. Clerics, paladins and blackguards may turn undead which causes the undead creature either to flee in fear or be destroyed altogether.
See also: Category:Undead