Undead are immune to many things. I'd like to create a list of all their immunities. This is what I have so far:
Mind altering effects, like Sleep, Confusion, Horror, Stun, etc.
Some spells: Slay Living, Cause Wounds spells, Vampiric Touch.
Not sure about the spells that create gases. I just tried Stinking cloud on a vampire and he was immune. However, Death Fog did damage him. Also not sure about the stun effect. I think I've seen undead stunned by darts of stunning.