Naxxramas

Naxxramas was an Undead Scourge necropolis floating over the Eastern Plaguelands near the city of Stratholme and later in the Dragonblight near Wintergarde Keep. During both encounters with it, members of the Argent Dawn, Argent Crusade, Horde and Grand Alliance faced forces under command of the Lich Kel'thuzzad.

Though not a well known fact, after Kel'thuzzad's defeat in the Plaguelands his Phylactery was not delievered to the Argent Dawn forces at Light's Hope but instead to Father Inigo Montoy who instead of destroying it, used it to resurrect the Lich during the War against the Lich King



Adapted from wowpedia