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 introduces what might be this season's Big Bad: a menacing figure named Adar.

If you're a massive Middle-earth aficionado but find yourself flummoxed at this most recent development, don't fret. Adar is not a character from J.R.R. Tolkien's works — he's an...