The first Dark Lord in J.R.R. Tolkien’s The Silmarillion is Morgoth.
Morgoth was originally called Melkor, one of the most powerful of the Ainur created by Eru Ilúvatar. He rebelled against Eru’s design and sought power over Arda. After his rebellion, the name Morgoth, meaning “Black Foe of the World,” was given to him by the Elf Fëanor.
Morgoth is the primary enemy of the First Age stories. He steals the Silmarils, destroys the Two Trees with Ungoliant’s help, and establishes strongholds such as Angband. His wars against the Elves and Men shape much of the history recorded in The Silmarillion.
Sauron is often confused with Morgoth because Sauron later becomes the central Dark Lord of The Lord of the Rings. In Tolkien’s hierarchy, however, Sauron begins as a servant and lieutenant of Morgoth. After Morgoth’s defeat and expulsion from the world at the end of the First Age, Sauron rises as the principal enemy of the Second and Third Ages.