Naxxanar is a nerubian Scourge necropolis floating above the Temple City of En'kilah in the Borean Tundra. It is the base of operations for the Scourge in the Borean Tundra and is the location of Prince Valanar, the local Scourge overlord.
Both the Alliance and Horde have quests to enter Naxxanar and kill Prince Valanar. Thassarian, first of the Alliance death knights, leads a group to the roof of the necropolis to face off against Valanar (aka Counselor Talbot of Valiance Keep) to free his sister, Leryssa, from the Blood Prince's mind control. The Horde, at the behest of Chieftain Wintergale of Taunka'le Village, find Valanar in the inner chamber of Naxxanar with his lieutenants, Vanthryn the Merciless and Luthion the Vile. To enter Naxxanar you need to kill High Priest Andorath, High Priest Naferset and High Priest Talet-Kha. After gathering their scrolls you receive the keywords "Shabtir bakh sokhen".