Basic Undead Race Traits

Then there were the basic special qualities which all undead possessed:
Immunity to critical hits, mind-affecting, poison, disease, sleep, paralysis, death, and energy drain effects. Resistance to necromancy and biological penalties. Undead did not need to breathe, eat, or drink. They were healed by negative energy and had [Dark Vision].
Of course, they had weaknesses too, like Good, Light, and Holy Vulnerability IV, [Bludgeoning Vulnerability V], [Vulnerability to Holy and Good Consecrated Areas II], double damage from fire, and so on.

- Overlord; Vol 1; Chapter 2