MorgothA fictional character from J. R. R. Tolkien's universe, Middle-earth, Morgoth Bauglir (Morgoth means 'Black Enemy', Bauglir is 'The Constrainer'), originally Melkor, was - to begin with - the most powerful of the Valar, and he contended with Eru himself in the Music of the Ainur. If he must be assigned a dominion (like Ulmo and water, Varda and stars, etc.), he would be the Vala of Knowledge (Tolkien was always skeptical of science).
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Melkor ("he who arises in might") was jealous of Eru already before Arda was created, and wanted to be king of other wills himself. When Eru revealed the results of their song to the Ainur (Arda, as it was), Melkor was one of the first to descend into it, mainly from this desire.
Melkor fought with the other Valar for a long time for the control of Arda. While he was the single most powerful Vala, he was not able to stand up to the combined forces of the other, lawful Valar. However, these were busy ordering the new world, creating the mountains, the sky, the earth, the waters - so the fight was not even.
He was held at bay by the aid of Tulkas, who came late to the party, and the Valar ordered Arda to their pleasing. Melkor was only biding his time, however, so when the Valar finally rested, he and his followers (downfallen Ainur, like Sauron and the later Balrogs) attacked their dwellings and destroyed their Two Lamps (precursors to the Two Trees and the sun and the moon).
The Valar then retired to Valinor in the West, and Melkor held dominion over Middle-earth from his fortress of Utumno in the North. His reign ended, however, after Eru awoke the Elves in the East of Middle-earth, and the Valar resolved to rescue them from him. They made immediate and devastating war on him, and he was brought to Valinor in chains to serve a term in the Halls of Mandos for three Ages.
It was after this sentence was ended, and he used his newfound freedom to corrupt the Noldor (a people of the Elves who had relocated to Valinor) and steal the Silmarils, that Fëanor of the Noldor first named him Morgoth, "dark destroyer of the world". With the aid of Ungoliant he also managed to destroy the Two Trees and bring darkness to Valinor, before he fled.
Back in Middle-earth, he took up his reign in the North again, this time in Angband, which had not been destroyed as thoroughly by the Valar as Utumno had. This time however, there were Elves and after a time also Men and Dwarves who resisted him, so he was not the sole ruler of Middle-earth.
However, after building his strength, he soon dispatched his enemies, one by one, through violence or treachery, until only isolated pockets of resistance remained (such as the strongholds of the Dwarves and the Havens of Mithlond (check this w sources). His mastery was again complete.
But it was not to last. This time the part Elf, part Maia, part Man Eärendil managed to plead with the Valar until they agreed to send an army to vanquish Morgoth. This time, the Valar themselves did not go, but many of the Maiar went, and most of the Calaquendi (Elves living in Valinor) ferried over into Middle-earth by the ships of the Teleri (another people of the Elves).
This time, he was utterly defeated, and his punishment was final. He was shut outside the gates of the world forever (or at least until the rumored Final Battle when he supposedly returns to fight a united army of Valar, Maiar, Elves and Men).
One legend of Middle-earth suggests that in the Last Battle, Morgoth will be slain by Turin Turambar, who will return from the dead to defeat him. This legend was included in one of Tolkien's many notes on The Silmarillion, and it was published in The Shaping of Middle-earth (Book 4 of the History of Middle-earth series).
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