Morgoth is known as the first Dark Lord of Middle-earth.
Morgoth was originally called Melkor, one of the powerful Ainur who entered the world in Tolkien’s creation story. He rebelled against the design of Eru Ilúvatar and became the primary enemy of the Elves and Men during the First Age.
Sauron originally served Morgoth and later became the central Dark Lord of the Second and Third Ages. This distinction is one of the most important ways to separate the chronology of Tolkien’s mythology: Morgoth dominates the earlier legends, while Sauron rises after Morgoth’s defeat.
Saruman is a corrupted wizard, and the Witch-king is the chief of the Nazgûl. Neither is the first Dark Lord. Tolkien’s later writings also use the name Melkor for Morgoth before his fall.