Mairon, also known as Sauron, was a fallen Maia. He created the One Ring and was second hand of Melkor. When Emlokr was defeated by the Valar, Mairon became the Dark Lord and began his quarrel to conquer middle-earth; to achieve this, he created the Rings of Power.
He was defeated by an alliance of Elves and Men in the second age, commanded by Gil-galad and Elendil. Isildur was able to cut Sauron's finger taking away the one ring from him, effectively destroying Sauron's corporeal form, but the Maiar's spirit perdures, and even though the war was won, Sauron was not defeated entirely, and because men couldn't destroy the ring, Sauron's spirit lay dormant for centuries, and then he returned to power by the end of the third age, although this time he would be pernanently defeated because the ring of power was destroyed by the hobbit Frodo Baggins, right at the end of the third age.
Sauron was also called Gorthaur the Cruel by the Elves of Beleriand in the First Age.
