Morgoth (meaning "The Dark Enemy") was originally named Melkor ("He Who Arises in Might"). The name Morgoth is the more common version of Morgoth Bauglir (Bauglir meaning 'The Contrainer'). At the creation of Eä, he was the most powerful of the Ainur (angelic beings). He contended with Eru (the Creator) in the Music of the Ainur, increasingly attempting to alter the music to his own design, but only ended up perverting its harmony. Melkor was the first to bear the title the Dark Lord and is the principal antagonist in the book The Silmarillion. As a fallen "angelic" being, he is largely based on Christian teachings about Satan (Satan is a seraph, the most powerful servants of God, and the Ainur also are the most powerful servants of Eru). Morgoth drew about him an army of Balrogs and other assorted demons, who were Maiar whom he managed to trick or corrupt, and made war on Middle-earth and the Children of Ilúvatar, but was defeated by the Valar and was overthrown. He was first taken away in chains, and later cast out of the circles of Arda after being weakened by dispersing his essence into the fabric of the world, marring it permanently (see Arda Marred).
Even before Arda was created Melkor was jealous of his creator, and he wanted to be king of other wills himself. He spent a long time looking for the Secret Fire, either unaware or uncomprehending that this was a quality exclusive to Eru and his servants through him. During his search, which took him far away from Eru and his brethren, his thoughts gradually became less and less in harmony with their own, and this was the beginning of his downfall. During the Music of the Ainur Melkor directly competed with Eru's theme, and managed to draw many lesser Ainur to him. Melkor's competition with the themes of Eru only enriched the creation; however, when Ulmo thought of water Melkor tried to destroy it with immense heat and then immense cold, only achieving clouds and ice. When Eru revealed the results of their song to the Ainur (the material world, called Arda), Melkor was one of the first to enter 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 might of all other Valar combined. However, these were busy ordering the new world, creating the mountains, the sky, the earth, the waters, and were desperate to create order among Melkor's chaos — so the fight was not even.
Melkor was held at bay by the aid of Tulkas, who arrived late in Arda only to fight him, 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) attacked and destroyed the Two Lamps (precursors to the Two Trees and the Sun and Moon). In the process, Arda was plunged into darkness, and Almaren, the home of the Valar, was destroyed.
The Valar then retired to Valinor in the West, and Melkor held dominion over Beleriand from his fortress of Utumno in the North. Melkor at this time began diluting himself all throughout Arda: he managed to taint all matter with his essence, in essence marring all of the world. However, this weakened him so that he became in essence far less powerful. Melkor's reign ended after Eru awoke the Elves in the East of Beleriand, 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. During this war much of the north of Beleriand was destroyed. Unknown to the Valar, Melkor had already begun capturing Elves, turning them into Orcs—a process which continued during his capture under his servant Sauron.
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 the Morgoth, "The Dark Enemy of the World". With the aid of Ungoliant he also managed to kill Finwë, Fëanor's father and High king of the Ñoldor, destroy the Two Trees, and bring darkness to Valinor, before he fled.
Back in Beleriand, 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 Beleriand. Melkor had also become weakened because of his dilution throughout Arda, and as Morgoth, although by far overpowering the might of Elves, Men, and Dwarves combined, he could no longer achieve the deeds of which he had once been capable.
However, after building his strength (mainly enforced through his Orcs, which by now outnumbered the Ñoldor and Sindar), 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. Eärendil, descended from Elves, Maiar, and Men, managed to plead with the Valar until they agreed to send an army to vanquish Morgoth. This time, the Valar themselves did not go, in fear of destroying even more of Middle-earth in a devastating war, 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 (a people of the Elves akin to the Sindar).
During the ensuing War of Wrath, Beleriand and much of the north of Middle-earth was destroyed, but at the end Morgoth was utterly defeated, and his punishment was final. Because he had weakened himself so he could no longer resist the Valar. Melkor/Morgoth was executed, and his fëa was shut outside the gates of the world forever. Shut off from Arda where most of his essence remained he was unable to ever return (or at least until the rumored Final Battle when he supposedly returns to fight a united army of Valar, Maiar, Elves and Men). Morgoth's evil, however, remained in Arda Marred.
One legend of Middle-earth suggests that in the Last Battle, Morgoth will be slain by Túrin 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).
Melkor initially could take any shape, and at first had a fair appearance, like his fellow Valar. However after he had succeeded in alienating the Ñoldor from Valinor and stealing the Silmarils, he descended in the pits of Angband, and his shape eventually became that of the Dark Lord, the Morgoth: 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. That battle also saw Thorondor, the great Eagle, swoop down and scar the face of Melkor with his talons, a wound which also never healed. In battle he wielded Grond, the Hammer of the Underworld.
Melkor's powers were originally 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, as Morgoth he 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 emulated in miniature by his servant Sauron, by creating the One Ring.) When, at the end of the First Age, Morgoth was dragged out of Angband in chains, he could not even resist the army of Valinor.
Morgoth was very greedy and selfish, and he would destroy all that did not serve him if he thought that they were a threat to him. pity was beyond his understanding, as was courage, for he alone of the Valar knew fear.
As the mightiest dweller in Arda, Morgoth's chief servants were certain Maiar he corrupted: Sauron, later the Dark Lord of Mordor and his Chief Lieutenant; Gothmog, Lord of Balrogs and High-Captain of Angband; Glaurung, the Father of Dragons; and Ancalagon the Black, greatest of the Winged Dragons. Morgoth did not trust any of them and considered them all his inferiors.
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 independent 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 Atanatári (Fathers of Men) tried to forget spoke of a Dark Lord who led men to worship him, banning Ilúvatar from their hearts. The Atanatári 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 Ñoldor were defeated, he bound all Men in his service to the lands of Hithlum, forbidding them to stray from there. It seems that, once victorious, he would have destroyed them as he intended to destroy those whom they fought for him.
Morgoth played a large role 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.
Morgoth also begat the race of the Orcs by twisting and distorting Elves, and bred the Dragons, and he also originated the Trolls in perverted mockery of the Ents. His evil could only ruin and destroy, never create. His servants were always wretched shadows of the noble creations of the other Valar. He corrupted many of the Maiar to his service, among them the spirits of fire which would become known as the Valaraukar, or Balrogs.
Perhaps his greatest evil was creating the rebellion of the Ñoldor against Valinor, and the destruction of the Two Trees and the theft of the Silmarils. This led to the disastrous Oath of Fëanor, which all but annihilated the entire house of the Ñoldor in Middle-Earth.
Legends amongst Men also blame him for their Fall, which stripped them of their immortality, although this cannot be clearly proven. The Elves claim that death is the Gift of Eru to the Atani. Certainly, Morgoth 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.
Perhaps the most long-lasting and direct of all his accomplishments was the corruption of Sauron, originally a Maia of Aulë. Sauron was to become Morgoth's greatest servant, and in ages after his master's fall, would arise as a new Dark Lord, following the same terrible path, and completing the estrangement of Men and Elves which Morgoth had begun.
Names and Titles
His name among the Ainur (used by both servant and enemy) was recorded as Melkor, in an old form Melkórë, which is actually Quenya for "he who arises in might". His original Valarin name is not recorded. The Sindarin form of this name was Belegur, which was never used except in the altered form Belegurth, meaning Great Death. The oldest name known, from Common Eldarin, was *mbelekôro.
After the Darkening of Valinor he was by the Elves never again called by his old name, but always called the Morgoth, the Dark Enemy. Other titles given to him were the Dark Lord, the Great Enemy, the Dark Power of the North, the Black Foe, and the Lord of the Dark. His oldest name among them was the Dark Hunter, a name from legend given when he captured Elves, to create the race of Orcs, and tried to discredit Oromë.
Men called him the Dark King of Angband or the Dark King. The Dwarves' name for him is unknown, and the Hobbits do not seem to be aware of his existence.
Morgoth and The Morgoth differed in meaning: "The Morgoth" was a term given to the person of Melkor in his complete power over the matter of Arda: therefore Dragons, Trolls, Orcs, and even Angband were in a way part of "The Morgoth", but not part of Morgoth. "The Morgoth" still held all the power Melkor had held of old, and was by far the most powerful being in all of Arda, but the incarnated Morgoth had lost so much power that he had almost become an equal of the greatest of the Elves: whereas Melkor would have been able to destroy Fingolfin without effort, Morgoth actually had to duel with the Elf-lord. While Melkor/Morgoth was eventually exiled by the Valar, the only way to destroy "The Morgoth" would be to completely destroy all of Arda and render it anew: a task the Valar could not do without also destroying the Children of Ilúvatar and therefore unthinkable.
Other Versions of the Legendarium
Previous versions of his name in early Tolkien's writings (see: The History of Middle-earth) are Melko, Belcha, Melegor, Meleko and others.
In the early stories, Melkor was much less powerful. He is described as being equal in power to Manwe. His power increased in later revisions of the story until he became the most powerful of the Valar, and then (in a late essay), more powerful than all of the Valar combined. In addition, late essays make Melkor the source of all evil and corruption in Arda. The published Silmarillion represents an earlier conception of Melkor's power: there is hardly any trace of Morgoth's marring of all Arda by diluting himself throughout it.