Undead is a special ability in Dominions 5. It represents a class of unit that can only be lead by special commanders and are rendered susceptible to banishment.

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.