In The Silmarillion he is described as a maia, a divine being, who was corrupted to evil by the first dark lord Morgoth and became one of his chief followers. He is the primary antagonist of The Lord of the Rings, in which he is the dark lord bent on controlling all of Middle-earth. His power is bound with the One Ring and he was finally defeated with its destruction.
He took on many forms with many names throughout his existence:
Mairon, his original name from before he became evil
Annatar, a light persona he took on to deceive Celebrimbor's people into forging the Rings of Power
The Necromancer, a name he was given when he haunted the forest of Mirkwood
