Sauron began his career in the First Age of Arda as a lieutenant of Morgoth. In later ages, he was the supreme dark lord in his own right. He made nine rings for mortal men which turned them into the Nazgûl, bound to his control under his own ring, made in secret.

When Ar Pharazon became so mighty that he overthrew Sauron's armies, Sauron allowed himself to be captured and corrupted Ar Pharazon and the high-men of Numenor into worship of Morgoth. They built the greatest fleet (probably of Spell Jammers) ever, and assaulted Valinor, where they were entombed till Dagor Dagorath.

Sauron also is expected to return at Dagor Dagorath. Recently the Cult of Sauron has begun the search for the Crucible and Avatar of Sauron to hasten that return.

External Sources: Tolkien Gateway on Sauron