From The Heroes of Might and Magic III wiki

All of the units from the Necropolis are undead, as well as the Mummy.

Undead units are unaffected by morale, and are immune to:

• all Mind spells
• Curse
• Bless
• Resurrection
• Death Ripple
• Azure Dragon's Fear
• Ghost Dragon's Aging
• Zombie's Disease
• Mighty Gorgon's Death Stare
• Vampire Lord's Life Drain
• Wyvern Monarch's Poison
• Lich's/Power Lich's Death Cloud (splash)
• Pit Lord's Demon Summoning Resurrection.

Undead units can be resurrected by the Animate Dead spell. All morale-affected units suffer -1 morale when fighting together with undead.