Undead is a special trait in Dominions 5. It represents a class of unit that can only be lead by special commanders, and that are susceptible to Banishment and its variants.

Undead units generally have at least partial darkvision, high cold resistance, and high poison resistance. Most of them need not eat, and all are unaffected by disease. They also tend to have high Max Ages that make Decay less serious for them.

Undead generally have high Morale, generally keeping them from routing as long as they have a commander with Undead Leadership on their side. Some, such as Longdead and Soulless, are mindless and will never rout; they will instead stop moving and fall to pieces on the battlefield, if left without undead leadership. Note that only some Undead behave like this; many types, such as Lictors and Ghosts, have minds and follow normal morale rules.

Demons are technically undead, but have different vulnerabilities.

From the manual:

Undead units are subject to banishment. Undead and demon units require undead leadership, which is generally conferred by skill in Death magic (or being an undead commander). Zombies aren’t going to take orders from just anybody. A squad that mixes undead and non-undead gets a -1 morale penalty, for obvious reasons.

Being undead grants no inherent protection against fatigue and does not inherently prevent drowning, although many undead also have the amphibious or poor-amphibian special abilities.

In addition to banishment, undead can be targeted by some particular spells like Solar Rays and Undead Mastery.