Dark Enemy of the World

From the first times after the world's creation, Melkor had been at war with the other Valar, misshaping and marring all that he touched. When the first Elves awoke, he sent his creatures to torment them, and to protect them the Valar sent a great force against Melkor and took him captive. He spent three ages imprisoned in the Halls of Mandos, but after his release, he plotted revenge against the Elves, many of whom had also travelled to Valinor.

As Melkor's plans came to fruition, the Two Trees were destroyed, and Valinor was plunged into darkness. Slaying Finwë the father of Fëanor, Melkor escaped from Valinor into Middle-earth with the Silmarils. Filled with rage, Fëanor assembled the Noldor and set out in search of revenge, swearing an Oath to recover the Three Jewels. At that time, he gave Melkor a new name, Morgoth, 'Dark Enemy', and by that name he is known in all the later tales of Beleriand.

This title as a meaning of the name Morgoth, in the full form 'Dark Enemy of the World'. In a similar vein, when describing Fëanor's original coining of the name Morgoth, the Silmarillion interprets it as 'Black Foe of the World'. This might be taken to mean that 'of the World' was part of the meaning of the name Morgoth, but this is not in fact the case.

Morgoth derives from the Elvish elements mor ('dark', 'black') and goth (from kot-, 'enemy', 'foe'). In other words, Morgoth literally means 'Dark Enemy' or 'Black Foe'. The conventional additional element, 'of the World', emphasises the Dark Lord's desire for mastery, and his enmity for the defenders of Arda, but it isn't actually present in the elements that make up the name Morgoth.