"Perfidious Eldar! They ruled the stars and must now sift through the dust of their once fabulous empire. For all their intellect and mysticism, they could not contain the beast or tame the savage monsters of the shadow. Why should we pay attention to them?"
The Aeldari, or Eldar as they have long been known to outsiders, are an ancient and highly evolved species of humanoid xenos whose vast empire once spanned the vastness and breadth of the known galaxy. In its day, the Aeldari Empire was unparalleled in the galaxy, spreading across both realspace and the webway. They considered themselves masters of the stars, and each Aeldari was born a powerful psyker, whose abilities were often used to both create and power their near-magical technology.
Even death was no obstacle, for when the spirits of the Aeldari finally left their mortal bodies, they peacefully dissolved back into the Immaterium to be reborn in new forms, for the warp then did not thirst for Aeldari souls as it does now. But ten millennia ago, the Aeldari's overbearing pride and their lapse into amoral and hedonistic practices led to a cataclysm that nearly wiped out their kind and led to the birth of the Chaos God Slaanesh, known to the Aeldari as "She Who Thirsted".
Despite their limitless power, the heart of their civilization was ripped out by this self-inflicted cataclysm, forcing many of the surviving Aeldari to adopt other survival strategies to protect their souls from being devoured by She Who Thirsts. These strategies shaped the nature of the surviving Aeldari cultures. In the 41st Millennium there are five major subcultures or relatives of the Aeldari species: the Asuryani or Craftworld Aeldari, the Drukhari, the Harlequins, the Exodites, and the Ynnari.
There are also bands of independent Aeldari who survive by raiding the shipping and trades of other species, many of whom are Asuryani who have emerged from their people's often rigid and restrictive culture. They are known as Asuryani Outcasts and Aeldari Corsairs. The Asuryani fled the original homeworlds of the Aeldari Empire in large starships the size of a continent called Craftworlds. These craftworlds are now scattered across the galaxy, connected only by the Webway's labyrinthine corridors. The Asuryani attempt to protect themselves from the hunger of She Who Thirsts using a soul-capturing psychocrystalline technology known as the Spirit Stones and a strict code of conduct called the Asuryani Path.
The Drukhari are the descendants of the Aeldari who originally settled the Webway, specifically the ancient city of Commorragh. The Drukhari protect themselves from the hunger of Slaanesh by following what the Asuryani call the "Path of Doom"; these pirates revel in the physical and mental pains of others, for only by feeding on the psychic remnants of suffering can they ward off the slow consumption of their own souls by you who thirst.
The Harlequins are the servants of the laughing Aeldari god Cegorach, who fled into the webway at the time of the fall of the Aeldari and were protected from the thirst of the Slaanesh by their god's power. The Harlequins are the custodians of the Aeldari's knowledge of chaos, known as the Black Library, and they are the only Aeldari left free to roam among all other factions. Their goal is to preserve among those around them the knowledge of the Aeldari mythological cycles, which form the basis of all Aeldari culture.
The Exodites are those Aeldari who fled the old homeworlds before the Fall and chose to settle on verdant, uninhabited planets known as the Virgin Worlds. The Exodites are protected from the soul thirst of Slaanesh by merging their souls after death with the world spirits of their home planets, and from the darker impulses of the Aeldari psyche by adopting a lifestyle characterized by more harshness and less labor-saving technology than any of their brothers.
The Ynnari, also known as the Reborn, are an Aeldari religious sect composed of all other factions who serve the partially awakened Aeldari god of the dead Ynnead. Led by Ynnead's high priestess and chosen prophetess, Yvraine, daughter of shadows, the Ynnari arose just before the birth of the Great Rift and the beginning of the Indomitus Era.
The Born Again seek nothing less than the full awakening of Ynnead, who will then battle Slaanesh, destroy the Chaos God, and restore the Aeldari people to their lost greatness. Ynnari need no longer fear Slaanesh's thirst, for upon death they become one with Ynnead.