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.
Internal history
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.
The Rings gave power to the bearers, but were, in secret, designed to enslave the Bearer to a Master Ring: the One Ring which Sauron himself forged in the fires of Orodruin, the Mount Doom. Men were indeed enslaved, but the Elves sensed the danger and took off their Rings. A great war ensued for the freedom of Middle-earth. Eregion was destroyed by the servants of Sauron, but the Men of Númenor came to the help of the Elves and drove Sauron back to Mordor in 1701. After this reversal, Sauron sought instead to build power in the eastern countries and left the Westlands in peace for many centuries.
When Ar-Pharazôn usurped the throne of Númenor in 3255 S.A., he saw Sauron's growing eastern kingdom as a threat to his power in Middle-earth. Building and equipping a mighty fleet, he sailed for Middle-earth, and landed at Umbar, to the south of the Mouths of Anduin. Ar-Pharazôn demanded that Sauron should submit to his authority, and seeing that the might of the Númenóreans far outstripped his own, Sauron agreed. However, Ar-Pharazôn with the greatest part of his people was seduced by Sauron and Númenor was brought to fall. With his greatest opponent out of the way the Dark Lord started a new war.
Although he had the power of his long-built armies and the One Ring, Sauron's own dominion in Middle-earth had decreased during his absence while the Elves´ High King, Gil-galad, had expanded his influence. If Sauron had waited longer in attacking his enemies while focusing on reestablishing his power, he would have surely defeated them all in time. But Gil-galad and King Elendil formed the Last Alliance, and after a long war they defeated the Dark Lord of Mordor. The One Ring was taken from Sauron and he disappeared.