The ancient Empire of Nazjatar, the great capital city of the naga, is built into the walls of the Rift, near the ruins of Suramar and close to Deepholme, directly underneath the Eye of the Maestrom. It is where most naga dwell, and their empress holds court within a beauteous palace even larger than said monarch's former home on land.
Somehow, against all odds, Azshara and her Highborne elite managed to survive the ordeal. Tortured and twisted by the powers they had released, Azshara and her followers were dragged down beneath the raging sea by the Well's implosion. Cursed - transformed - they took on new shapes and became the hateful serpentine naga. Azshara herself expanded with hate and rage, becoming a massive monstrosity, reflecting the wickedness and malice that had always hidden within her core.
There, at the bottom of the Maelstrom, the naga built for themselves a new city from which they would rebuild their power. It would take over ten thousand years before the naga would reveal their existence to the surface world, in the form of a rudimentary expedition led by Lady Vashj.