Naxxramas is an introductory level-80 raid dungeon floating aboveDragonblight in Northrend. It is a Scourge necropolis, the seat of the Arch LichKel'Thuzad. The 40-man classic version of Naxxramas is no longer available, having been permanently closed with the release of the Wrath of the Lich King expansion.
The original incarnation of the dungeon was regarded as the most difficult pre-Burning Crusade raid, requiring 40 well-equipped, skilled players to complete. In the World of Warcraft: Wrath of the Lich King expansion, Naxxramas was moved to Northrend and retuned as an entry-level raid dungeon for level 80 players. Like all other Wrath raid dungeons, Naxxramas has both 10-man and 25-man versions.
The Carrion Fields,Dragonblight
Years ago, the crypt lord Anub'arak led an army of undead warriors into the ancient nerubian ziggurat now known as Naxxramas. The Scourge forces overran the citadel, and under the will of the Lich King its corridors were transformed into a potent war machine. Dark magic tore the fortress from its subterranean home and lifted it into the sky. Hidden behind a thick layer of clouds, Naxxramas grew in strength. At the Lich King's bidding, the burgeoning wrath within the necropolis was unleashed upon the world.
It was then used as Kel'Thuzad's base of operations as he spreads the plague throughout Lordaeron. Other sources say Obrahiim, the Traveler, a great skeletal architect, was pivotal in the conception of Naxxramas, hinting that it was altered or retooled. See the Compendium of Fallen Heroes for more details.
Due to his fighting a war against the Scarlet Crusade, the Argent Dawn, the Forsaken and the Humans of the Alliance, as well as constant incursions of adventurers from every race and nation into the Scourge-controlled Plaguelands on a daily basis, the forces of Kel'Thuzad have been severely taxed to maintain the security of the necropolis. But now the gates of Naxxramas are open, and Kel'Thuzad's new forces are rapidly sweeping away all opposition to the Scourge.
Blizzard has posted a short back story for Naxxramas on the official site: Road to Damnation
Above the war-ravaged wastes of the Dragonblight, Naxxramas has returned with renewed fury. Kel'Thuzad, once again seated in the frigid heart of the necropolis, has laid siege to the Alliance fortress of Wintergarde Keep. As factions from around the world converge at the gate to Icecrown, the war against the Scourge enters its most pivotal hour. It is within Naxxramas that the tide of battle could turn against the Lich King... or irrevocably in his favor.
Naxxramas is a giant necropolis floating over Wintergarde Keep, in Dragonblight. It has the questionable honor to serve as the seat of one of the Lich King's most powerful officers, the dreaded lich Kel'Thuzad.
The instance has been redesigned for 10 and 25-man raids. It consists of four different wings which, when cleared, grant access to the Frostwyrm Lair (a fifth and final wing). The first four wings are, in order of ascending difficulty: Arachnid, Plague, Military, Construct. Progression is non-linear - raids may select their own path to the final lair.