Naga History

The naga are former highborne night elves who mutated into vengeful humanoid sea serpents. While many remain on Azeroth to attempt to reclaim the surface world, a large group followed the power-mad betrayer, Illidan Stormrage, into Outland and proceeded to claim the watery areas of the shattered land.MG 105 In addition to their natural longevity, a number of naga develop mutations that increase their life spans even further. Some are immortal.

War of the Ancients
Naga are sinister, ophidian humanoids that dwell beneath the ocean. Theirs is an ancient and mystical culture, for ten thousand years ago they were Kaldorei.
Ten thousand years ago, during the reign of Queen Azshara, there was an elite magic-using sect known as the Highborne, who dabbled in magics that many other elves considered reckless and irresponsible[citation needed] by drawing upon the power of the Well of Eternity. The high-borne elves enjoyed great power and comfort before the War of the Ancients.MoM 66 During that conflict, the Highborne sought to abuse arcane magic to bring demons to the world. This action led to the War itself, in which the night elves, at great cost, defeated the Burning Legion. The magical backlash shattered the world, imploded the Well of Eternity, and blasted the Highborne’s capital city to the bottom of the ocean.MG 105 Many were cast far into the flooding depths of the great cataclysm that sundered the land and destroyed the Well of Eternity. Many Highborne joined their city on the ocean floor, and would have drowned if not for a curse that had turned them into water-breathing naga.[1] While many died, it was Queen Azshara's quick thinking that saved them. She managed to make a pact with the Old Gods on behalf of her people. With the Old Gods'Sund 332-333 curse they started to change. Since then they have further been twisted by the residual cascading energies of the Well of Eternity[2] They took on new shapes, new powers and became the hateful, serpentine naga.
When the naga awakened, they set about building a new life for themselves on the ocean depths, within the abyssal sea trench beneath the swirling waters of the Maelstrom they built their new city, Nazjatar. Azshara herself expanded with hate and rage, becoming a massive monstrosity, reflecting the wickedness and malice that had always hidden within her core. For 10,000 years, these elves of the abyss dwelt in a dark and powerful empire. Pagodas and balconies line the deep trenches, glimmering lights picking out the shapes of cities in the cold black. Sinuous figures drift and twist along the passageways, their elven origins difficult to discern.MoM 66