Eregion was populated by Noldorin Elves, and for a while ruled by Galadriel and Celeborn, until these left for Lothlórien on the other side of the Misty Mountains. It was the only Elven kingdom in all of Middle-earth to live in harmony with a Dwarven civilization, trading freely with the Dwarven kingdom of Khazad-dűm, or Hadhodrond in the Elven tongue.
After Galadriel and Celeborn had left, Eregion was ruled by Celebrimbor, a descendant of Fëanor. under his rule the Elves of Eregion became friends with Annatar, Lord of Gifts, and created the Rings of Power. When Annatar was revealed as the Dark Lord Sauron the Elves of Eregion in vain tried to keep the Rings from falling in Sauron's hands, but only managed to rescue Vilya, Narya, and Nenya. Eregion was utterly destroyed, and its survivors fled to the land of Lindon, Lothlórien, and the refuge of Imladris (Rivendell).
In the Third Age, Eregion was a pleasant but unpopulated land.