Who is Adar in 'The Lord of the Rings: The Rings of Power'?

The third episode of The Lord of the Rings: The Rings of Power introduced audiences to the season's Big Bad: Adar (Joseph Mawle), a menacing elven figure who leads the orcs of the Southlands. His origins — and whether or not he's actually Sauron — were left somewhat unclear... until The Rings of Power's showstopping sixth...