Blizzard sent out the following

 

Champions of the Horde and the Alliance, gather your strength (and frost resist gear) and prepare to take on the horrors that await within the floating necropolis of Naxxramas. The latest content update for World of Warcraft ClassicShadow of the Necropolis is now live!
Heroes who have reached Honored status with the Argent Dawn will be able to venture into this epic 40-player raid, home to some of the Lich King’s most powerful servants and their treacherous commander, the fallen mage Kel’Thuzad.
For more information on everything coming with Shadow of the Necropolis, head to our blog.

Will you be playing?