Sauron is a fictional character in J. R. R. Tolkien's Middle-earth world, and appears as the main antagonist in The Lord of the Rings.
In the Second Age Sauron disguised himself as Annatar, the "Lord of Gifts". He came to the Elves of Eregion and taught them many things he had learned from his former master Aulë, which resulted in the forging of the Rings of Power. Together with Annatar the Elves made the Nine and the Seven. The Three were made by Celebrimbor without Sauron knowing, making them free of his taint. Sauron himself forged in secret the One Ring in the fire of the Sammath Naur of Mount Doom, pouring into it half of his power and lifeforce and will, to control and rule the other Rings and all of Middle-earth.
Towards the end of the Second Age there formed the Last Alliance of Elves and Men under the command of Gil-Galad and Elendil to fight against Sauron, in the War of the Last Alliance. After the Siege of Barad-dûr, Sauron killed Elendil, but Isildur picked up his father's sword Narsil and cut the Ring from his hand, and Sauron was defeated. But because part of his power and life was in the One Ring, Sauron continued to exist as long as the Ring remained. Over the millenia he managed to again gain power and form towards the later part of the Third Age.
The Lord of the Rings tells the story of the final conflict, the War of the Ring in the years T.A. 3018 and 3019. During this time he wares open war aganist the free peoples of Middle-earth, while searching for the One Ring. The Hobbit Frodo Baggins is sent out to take the Ring back to Mount Doom and destroy it by casting it back into the Sammath Naur where it was forged. In March T.A. 3019, the One Ring was destroyed when Gollum fell into the fires of the Sammath Naur while holding onto the One Ring, finally destroying it and its creator Sauron permanently.