Sauron

Sauron is the main antagonist of the Middle Earth Legendarium. He is the main antagonist of The Lord of the Rings, the overarching antagonist of The Hobbit, and the secondary antagonist of The Silmarillion.

He is the overarching antagonist of The Lord of the Rings: The Fellowship of the Ring and The Lord of the Rings: The Two Towers, and the main antagonist of The Lord of the Rings: The Return of the King.

He also serves as a flashback antagonist in The Hobbit: An Unexpected Journey, then the secondary antagonist of The Hobbit: The Desolation of Smaug, and the overarching antagonist of The Hobbit: The Battle of the Five Armies.

During the first age, Sauron served under the Valar Morgoth, but after his master's "banishment" Sauron then took up the mantel and title of Dark Lord, creating The One Ring to use as a tool for enslaving the free people's of Middle Earth during the Second and Third Age.