The Kaldorei, or "Children of the Stars" in their native tongue—more commonly known as Night Elves—are one of the oldest and most influential mortal civilizations on Azeroth. Millennia ago, they began as a nomadic tribe of dark trolls who settled along the shores of the Well of Eternity, a massive lake of pure, volatile arcane energy at the heart of the ancient continent of Kalimdor. Over generations, the cosmic energies of the Well fundamentally mutated their physical forms, granting them towering statues, luminescent silver eyes, violet skin, and functional immortality.
Developing a deep spiritual connection to the stars and the moon goddess, Elune, the Kaldorei rapidly built a magnificent, sweeping empire of shining marble cities. Under the tyrannical rule of Queen Azshara and her elite caste of sorcerers, the Highborne, the Kaldorei mastered arcane magic to an unprecedented degree. However, Azshara’s reckless, obsessive manipulation of the Well’s power ultimately drew the attention of Sargeras and the Burning Legion, plunging the world into the catastrophic War of the Ancients.
The War of the Ancients concluded with a desperate, apocalyptic victory. To stop the demonic invasion, the Kaldorei and their allies were forced to destabilize and destroy the Well of Eternity itself. The resulting magical implosion, known as the Great Sundering, tore Kalimdor apart, sinking eighty percent of the landmass beneath the ocean and creating the Maelstrom.
Shaken by the horrific cost of their arcane hubris, the surviving Kaldorei completely abandoned arcane sorcery under the leadership of the High Priestess @Tyrande Whisperwnd and the first mortal druid, @Malfurion Stormrage . They outlawed the practice of arcane magic on pain of death. To heal the fractured world, the Dragon Aspects planted a new World Tree, Nordrassil, atop a second, smaller Well created by the traitor @Illidan Stormrage . The bronze dragon Nozdormu bound the tree to the Kaldorei, granting them renewed immortality so long as the tree stood, while the green dragon Ysera bound their spirits to the Emerald Dream. Thus, the Kaldorei shifted from a decadent magocracy into a deeply reclusive, nature-worshiping society.
For the next ten thousand years, a period known as the Long Vigil, Kaldorei society operated under a unique cultural division of labor. The male Kaldorei largely embraced the path of druidism, entering centuries-long slumbers within the Emerald Dream to maintain the spiritual balance of Azeroth's nature.
In their absence, the defense and governance of their territory fell entirely to the female Kaldorei. @Tyrande Whisperwnd established the Sisterhood of Elune as the spiritual and political leadership of the nation, backed by the Sentinels—an elite, highly trained military force of fierce archers, huntresses, and glaive-throwers. Operating from the dense, shrouded forests of Ashenvale and Mount Hyjal, the Sentinels ruthlessly executed any outsiders who dared trespass into their sacred domains, keeping the night elven civilization shrouded in myth and mystery to the Eastern Kingdoms.
The isolation of the Kaldorei was shattered permanently during the Third War, when the Burning Legion returned to Azeroth alongside the Scourge. Human refugees led by @Jaina Proudmoore and the orcish Horde led by @Thrall arrived on the shores of Kalimdor, inadvertently clashing with the defensive Sentinels. Recognizing the existential threat of the Legion, @Tyrande Whisperwnd awakened @Malfurion Stormrage and the sleeping druids.
Ultimately, the night elves formed a desperate, historic truce with the Horde and the Alliance. In the climactic Battle of Mount Hyjal, @Malfurion Stormrage channeled the ancestral spirits of the forest through Nordrassil, creating a cataclysmic explosion that destroyed the demon lord Archimonde. While victorious, the cost was staggering: Nordrassil was severely damaged, and the Kaldorei permanently sacrificed their racial immortality and their immunity to aging and disease.
In the wake of this devastating loss, and facing territorial encroachment from the Horde in Ashenvale, the Kaldorei officially abandoned their historic isolationism and formally aligned themselves with the Alliance, establishing a new capital city, Darnassus, within the boughs of a second, unblessed World Tree named Teldrassil.