Sauron the Great

Much like Gandalf or Saruman, Sauron the Great was a servant of the West, a Maia.  He was corrupted in the early days of Arda when Melkor, later renamed to Morgoth, rebelled against Eru and fled to Middle-Earth to make his stronghold in Angband.  Sauron became one of Morgoth's lieutenants, a powerful spiritual being who before the sundering of his body walked among the Elves and Men.

In the Great War of the Elves and Men against Sauron, Sauron forged the One Ring to rule over the various races to which he had given lesser rings of power.  The One Ring held much of his power, but in the midst of the battle Isildur, Elendil's son, struck Sauron's hand and the Ring was severed from its master's hand.

Sauron shed his physical body and fled in spirit form to the shadows, nursing his strength and biding the time until he would rise again.  In time he moved his stronghold to the Southern part of Mirkwood, where he was known as the Necromancer.

After the resurfacing of the Ring among the Halflings, Sauron's power again waxed and his power was bent on finding the Ring.  The course of that search and the events that followed was recorded in the Lord of the Rings.  The Ring was destroyed at last in the fires of Orodruin, and Sauron's spirit was shrivelled and misshapen, becoming but a shadow of his former power.  He once again fled to the shadows, reduced to a mere shade, and was never heard from again.