The Chamber of the Avatar is the room for which the Tomb of Sargeras is named. It is a gigantic titan vault in which the Fallen Avatar can be found, crudely nailed to a titan machine, held in place by beams of light, and guarded by the fully-demonic Maiden of Valor. Kil'jaeden reactivates the inert avatar, filling it with a crude sentience.
When the avatar fully breaks free of its bonds, the floor collapses into a giant lake of fel with the source of the Felstorm on its shore. The map calls this area Felstorm's Breach. The Eye of Aman'thul is used to close Kil'jaeden's portal to The Twisting Nether there after he is followed by Khadgar, Velen, Illidan, and adventurers to stop him once and for all.