Naxxramus

From wow.incgamers.com

Naxxramus was the End Game Instance before the expansion The Burning Crusade was released. It is for level 60+ characters and requires attunement with The Argent Dawn to enter.

It is a floating necropolis over Eastern Plaguelands, accessible through a transporter on the ground.

Naxxramus will move to Northrend when the next expansion Wrath of the Lich King is released.