Naxxramas

Naxxramas, the floating Necropolis, is is a 40-man raid dungeon, located in the northwest side of the Eastern Plaguelands. This citadel is the forward base of operations of the Scourge within Azeroth, containing some of the Lich King's most powerful servants, including the frost wyrm Sapphiron and the lich Kel'Thuzad. It is by far the most challenging raid that players have encountered, so would-be intruders are advised: be prepared for some of your toughest fights yet!