Undead

Undead is a vague term used to refer to any creature that has died and come back, either the soul (eg Ghost and Wraiths) or body (eg Zombie and Living Skeletons). Undead are either created by Necromancy or created when a soul is stuck in the material realms. They can't be killed, due to already being dead, and can be extremely dangerous, not because they're particularly strong or powerful but because they're very difficult to stop.