He was a Maia, an angelic being in the Legendarium, who was corrupted by Morgoth and became a Dark Lord bent on controlling the world of Middle-earth. He is the main antagonist of The Lord of the Rings trilogy and was ultimately defeated after the destruction of the One Ring.
He took on many forms with many names throughout his existence:
Mairon, his original Maia name
Annatar, a persona he took on to deceive the Elves into forging the Rings of Power
