Morgoth is the evil Vala who serves as the main antagonist in J.R.R. Tolkien’s First Age stories.
Originally called Melkor, he was the most powerful of the Ainur who entered Arda. His rebellion against Eru Ilúvatar and the other Valar makes him the first great enemy of Tolkien’s world. He seeks domination rather than merely serving another master, and he corrupts much of Arda through his actions.
Sauron begins as one of Morgoth’s servants and later becomes the central enemy of the Second and Third Ages. This is why readers sometimes identify Sauron as Tolkien’s ultimate villain, even though Morgoth predates him and is presented as the larger First Age threat. Morgoth is defeated and cast out of the world after the War of Wrath.