Dark Lord was the title of the two most powerful of the evil beings in Arda. The first and greater Dark Lord was the Vala (at first) known as Melkor and then as Morgoth; the second and lesser Dark Lord was the Maia, Sauron. Both Dark Lords were very influential in the history of Middle-earth. Eventually the two Dark Lords were overthrown and defeated, at the end of the First Age and at the end of the Third Age.
Behind the ScenesEdit
"Dark Lords" became something of a cliché in fantasy novels following the success of JRR Tolkien's The Lord of the Rings with the Dark Lord Sauron.
Later in the Silmarillion it is revealed that Sauron was the second Dark Lord and that the first was Morgoth--Sauron actually being a minion of Morgoth.