Tomb of Sargeras multi-wing dungeonEdit
Since in Naxxramas Kel'Thuzad is gathering the pieces of Atiesh, Greatstaff of the Guardian, he could be trying to use it to rebuild Sargeras dead corpse (the staff holds his hand after all) and then raise it to be servenat of the Scourge. Let's imagine if he has indeed succeded? With the staff completed, the Scourge sent an elite regiment of necromancers to the tomb.
Three wings composed this dungeon, and attunement for the second and third wings must be obtained by completing the easier wing. Completing the quests will give reputation with Kirin Tor (or whatever will it be called the ruler faction in Dalaran in the upcoming expansion)
A group of Nagas that lived in the abandoned city were kicked out the Scourge, and now dweel in an underwater cave and sewer system, linked to Suramar's prison. These nagas are ruled by the Warlord Tideslicer, Lady Jasha and the Water Elemental Hydrocrusher. You must kill them all to create a diversion for Kirin Tor to send spies to investigate the city.
The spirits and still usable corpses of the orcs and demons (killed when Gul'dan raised the city from the sea) were raised by the Scourge. Lead by the Death Knight Jason the Sunderer and the spirit of Gul'dan, it appears that for whatever reason the Scourge came, no one is allowed to see. You must kill him to get the key to the only available gate to Sargeras's resting place.
Tomb of Sargeras (40-man raid dungeon)Edit
Oops. You were too late. Sargeras's avatar has been already raised by the Necromancer Katrina the Raiser. Thank God you have lots of friends with you. The battle against Ghoul of Sargeras would be something like this:
Kill Katrina quickly as she summons waves of undead at non-stop rate. After that you can battle Sargeras with no interferences. Ghoul of Sargeras is the strongest and hardest raid boss in the game. However, he is killable. After you take him down, Soul of Sargeras apears and thanks you for freeing him from the Scourge. He asks you if you want to be his personal honor guard, with high-honor ranks in the Burning Legion. You refuse and he leaves, promising revenge.