In J.R.R. Tolkien’s mythology, the fallen Vala later called Morgoth was originally named Melkor.
Melkor is introduced in Tolkien’s legendarium as the most powerful of the Ainur, the spiritual beings created by Eru Ilúvatar. His desire to create independently and dominate others leads him into rebellion, making him the principal enemy of the early history of Arda.
The name Morgoth, meaning “Black Foe of the World” in Tolkien’s invented terminology, was given to him after he destroyed the Two Trees of Valinor and stole the Silmarils. Sauron served as one of his chief lieutenants but was not the same being.
Melkor and Morgoth are therefore two names for one character at different stages of his story. The distinction is central to The Silmarillion’s account of the First Age.