The Lord of the Rings is Sauron, Dark Lord and lieutenant of Morgoth. He is called this for fashioning the ruling ring, the one that holds dominion over all other Rings of Power.

Actually, Lord of the Rings is just one of his many nomenclatures. Here are a few others: the Dark Lord, the Black Hand, the Necromancer, the Ring-Maker, Annatar ("Lord of Gifts"), Gorthaur ("Terrible Dread"), the Red Eye, The Dark One, The Evil One, etc.

J.R.R. Tolkien Node