In J. R. R. Tolkien's mythology, the Dark Years is a term used in The Lord of the Rings for the time of Sauron's great and almost undisputed domination of Middle-earth, during which many peoples were enslaved or corrupted. It was also referred to as the Accursed Years, the Black Years, the Days of Flight, and the Dark Days. In the story, this period lasted from ca. SA 1693 to 3441, when Sauron was overthrown by the Last Alliance of Elves and Men. During this period, Sauron built the stronghold Barad-dûr, forged the Rings of Power, and fought the War of the Elves and Sauron.
Sauron was an emissary or servant of the real dark lord Melkor, or Morgoth. After the latter's defeat by the Valar, he was taken from his great fortress Thangorodrim, and taken to Valinor. There he stood trial and was cast into the Void. Sauron, however, escaped the Valar and hid in the east of Middle-earth. He returned in the year 1000 of the Second Age, though it would be centuries before he rose in real power.
As Annatar, he persuaded the Elves of Eregion to forge Rings of Power. The Elves, who trusted him, did so, and forged three mighty rings. Vilya and Narya, which passed to the Elven king Gil-galad, and Nenya which passed to the Elven lady Galadriel. Seven Rings were also made for the Dwarves, while Nine were made for Men.