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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 in the Blue Mountains, and minor refuges at the Mouths of Sirion and the Isle of Balar). 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).
Morgoth originally could take every shape, and at first had a fair appearance, like his fellow Valar. However after he had succeeded in alienating the Noldor from Valinor and stealing the Silmarils, he descended in the pits of Angband, and his shape eventually became that of a Dark Lord: gigantic and terrible to behold. During this time he lost the ability to change shape, and in effect became bound to this one, terrible form. His hands had been burned by the theft of the Silmarils, and never healed. During the one time he emerged to fight the High King Fingolfin, he was stabbed in the foot, and had a limp ever after.
Morgoth's powers were immense, at the very least equivalent to those of his brother Manwë's, and possibly greater. He shared a part of the power of all other Valar, but unlike them used this for his own gain. However, Morgoth dispersed his power over all of Middle-earth, tainting the very fabric of Arda itself with his will, and thereby lost most of his personal power. (This was later done by his servant Sauron in small, by creating the One Ring.) When at the end of the First Age he was dragged out of Angband in chains, he could not even resist the army of Valinor.
Mightiest dweller in Arda, Morgoth trusted nobody but himself. He had many servants, chief of them his lieutenants Sauron and Gothmog, lord of the Balrogs, but no-one he trusted or considered his equal.
Unlike the later Dark Lord Sauron, Morgoth's goal was not to dominate all of Arda, but rather to destroy it: because he could not control all of it he wished to eliminate it completely. He saw the Children of Ilúvatar (Men and Elves) as a direct threat, since they were independant souls he could not easily dominate.
At some point during the formation of Arda Morgoth either had an alliance with or had as a servant the spider-shaped entity Ungoliant, and during the Darkening of Valinor he formed an unholy alliance with her to destroy the Two Trees.
When the race of Men came about, Morgoth is hinted to have temporarily left his caves of Angband, and dwelled amongst them: ancient legends the Atanatari (Fathers of Men) tried to forget spoke of a Dark Lord, who led them to ban Ilúvatar from their hearts, and worship him. The Atanatari were those Men who repented and fled, but Morgoth ever after had many legions of fallen Men at his service. (Morgoth's Ring: "Tale of Adanel")
Morgoth even betrayed his own servants: after the Noldor were defeaten, he barred all Men in his service in the lands of Hithlum, forbidding them to stray from there. It holds no doubt that, once victorious, he would have destroyed them like he wanted to destroy those they were fighting for him.
Morgoth played a large rôle in the creation of Arda: from the start he fought his fellow Valar, even if all his schemes backfired. His unleashing of terrible cold on the waters of Ulmo brought about ice and snow, his horrible fires could not burn the seas, but created the clouds.
Yet he was partially victorious: he destroyed the Two Lamps, and distorted the original symmetry of Arda.
Legends amongst Men also blame him for their Fall, which stripped them of their mortality, although this cannot be clearly proven. Certain is that he created the fear of night and death in Men.
Because Morgoth dispersed his essence all over Arda, it is said that all of Arda outside of the Blessed Realm has some evil in it, this being the Morgoth-element.