The undead were once-living creatures, now animated by spiritual or supernatural forces. The term undead refers to all creatures or entities that function normally despite having terminated life functions. Most commonly known as zombies or ghosts, undead beings are most typically mindless, bloodthirsty fiends hostile toward any living thing that comes across their path.
The undead have appeared all throughout Azeroth, most commonly in nature as revenants or wraiths. They were used as a combat tool during the First War when the necrolytes of the Horde learned to animate corpses as skeleton warriors in their fight against the Alliance. Later, the studies of the necrolytes developed into necromancy, the form of magic used very heavily in the Third War when the Cult of the Damned spread the Plague of Undeath across Lordaeron.