Sauron was originally Mairon, a Maia of Aulë the Smith, created before history. In the Second Age, he invented the One Ring to help him attain dominance of Middle-earth. In the Third Age, after he lost the Ring, he never appeared openly, but was known for his Great Eye, the Eye of Sauron, which few could endure and which sought the world for his One Ring. He is the greatest worker of evil in Tolkien's writings after the demise of Morgoth at the end of the First Age.