Melkor was the most powerful of the Ainur, but turning to darkness, became Morgoth, the "great enemy", the ultimate antagonist of Arda, from whom all evil in the world of Middle-earth ultimately stems. Sauron, one of the Maiar of Aulë, switched his allegiance and became the principal lieutenant of Morgoth.
Morgoth was the principal agent of evil during the time of The Silmarillion, and his influence lingers in the world even after he was cast from the world into the outer void. Morgoth's example provided later ages a cautionary tale against pride, wrath, envy, lust for power, and greed — and the destruction these visit upon oneself and others.
The name Morgoth is Sindarin, one of Tolkien's invented languages, and means "Black Enemy"; Bauglir is also Sindarin, meaning "Tyrant" or "Oppressor". "Morgoth Bauglir" is actually an epithet: his name in Ainulindalë (the creation story of Middle-earth and first section of The Silmarillion) is Melkor, which means 'He Who Arises In Might' in Quenya, another of Tolkien's languages. But this too is an epithet since he, like all the Ainur, had another true name in Valarin (the language of the Ainur before Time), but this name was not recorded. The Sindarin equivalent of Melkor was Belegûr, but it was never used; instead a deliberately resembling name Belegurth, meaning 'Great Death', was employed, though still rarely.
The name 'Morgoth' did not arise until Melkor destroyed the Two Trees, murdered Finwë and stole the Silmarils in the First Age. Prior to this point, he was called only Melkor. But after these events, Fëanor, son of Finwë, dubbed him 'Morgoth', and the Elves called him thereafter by that name alone. (see "History" below)
Like Sauron, he had a host of other titles: Lord of the Dark, the Dark Power of the North, and Great Enemy. The Edain called him the Dark King and the Dark Power; the Númenóreans corrupted by Sauron called him the Lord of All and the Giver of Freedom.
Before the creation of Arda (The World), Melkor was the most powerful of the Ainur. He contended with Eru (God), via the Music of the Ainur. Melkor was jealous of Eru, and wanted to create and rule other wills himself. He spent a long time looking for the Secret Fire (the “Flame Imperishable”).
Unlike his fellow Ainu Aulë, Melkor was too proud to admit that his creations were simply discoveries wholly made possible by, and therefore “belonging” to, Eru. Instead, Melkor aspired to the level of Eru, the true Creator of all possibilities.
During the Great Music of the Ainur, Melkor attempted to alter the Music and introduced what he believed to be elements purely of his own design. As part of these efforts, he drew many weaker-willed Ainur to him — creating a counter to Eru’s main theme. Ironically, these attempts did not truly subvert the Music, but only elaborated Eru’s original intentions: the Music of Eru took on depth and beauty precisely because of the strife and sadness Melkor’s disharmonies (and their rectification) introduced.
Since the Great Music of the Ainur stood as template for all of history and all of material creation in the Middle-earth cycle (it was first sung before Time, and then the universe was made in its image), there was an aspect of everything in Middle-earth that came of Melkor’s meddling – everything had been "corrupted."
Each of the Valar was attracted to a particular aspect of the world that became the focus of their powers. Melkor was drawn to terrible extremes and violence — bitter cold, scorching heat, earthquakes, rendings, breakings, utter darkness, burning light etc. His power was so great that at first the Valar were unable to restrain him. Arda never seemed to achieve a stable form until the Vala Tulkas entered Eä and tipped the balance.
Driven out by Tulkas, Melkor brooded in the darkness at the outer reaches of Arda until an opportune moment arrived when Tulkas was distracted. Melkor re-entered Arda and attacked and destroyed the Two Lamps, which at the time were the only sources of light. Arda was plunged into darkness, and the island of Almaren, the first home of the Valar on Earth, was destroyed in the violence of the lamps' fall.
After the fall of the Lamps, the Valar withdrew into the land of Aman in the far West. The country where they settled was called Valinor, which they heavily fortified. Melkor held dominion over Middle-earth from his fortress of Utumno in the North.
Melkor’s first reign ended after the Elves, the eldest of the Children of Ilúvatar, awoke at the shores of Cuiviénen, and the Valar resolved to rescue them from his malice. The Valar waged devastating war on Melkor, and destroyed Utumno. Melkor was bound with a specially forged chain, Angainor and brought to Valinor, where he was imprisoned in the Halls of Mandos for three ages.
In the account published in The Silmarillion, Melkor had captured a number of Elves before the Valar attacked him, and he tortured and corrupted them, breeding the first Orcs. Other versions of the story (written both before and after the published text) describe Orcs as corruptions of Men, or alternatively as soulless beings animated solely by the will of their evil lord. This last version illustrates the idea of Morgoth dispersing himself into the world he marred.
Upon his release, Melkor was paroled to Valinor, though he was not trusted by some of the Valar. He made a pretence of humility and virtue, but secretly plotted harm toward the Elves, whose awakening he blamed for his defeat. The Noldor, wisest of the three kindreds of Elves that had come to Valinor, were most vulnerable to his plots, since he had much knowledge they eagerly sought, and while instructing them he also awoke unrest and discontent among them. When the Valar became aware of this they sent Tulkas to arrest him, but Melkor had already fled. With the aid of Ungoliant, a dark spirit in the form of a monstrous spider, he destroyed the Two Trees of Valinor, slew the King of the Noldor, Finwë, and stole the three Silmarils, jewels made by Finwë’s son Fëanor, which were filled with the light of the Trees. Fëanor thereupon named him Morgoth, "Black Foe of the World", and the Eldar knew him by this name alone afterwards.
Morgoth resumed his rule in the North of Middle-earth, this time in Angband, a lesser fortress than Utumno, but not so completely destroyed. He rebuilt it, and raised above it the volcanic triple peak of Thangorodrim. The Silmarils he set into a crown of iron, which he wore at all times. Fëanor and most of the Noldor pursued him, along the way committing the Kinslaying and incurring the Doom of Mandos. On arriving in Beleriand, the region of Middle-earth nearest Angband, the Noldor established kingdoms and made war on Morgoth. Soon afterwards, the Sun and the Moon arose for the first time, and Men awoke if they had not done so already. The major battles of the ensuing war included the Dagor-nuin-Giliath (Battle Under the Stars, fought before the first rising of the Moon), Dagor Aglareb (Glorious Battle), Dagor Bragollach (Battle of Sudden Flame) at which the long-standing Siege of Angband was broken, and the battle of Nírnaeth Arnoediad (Unnumbered Tears) when the armies of the Noldor and the Men allied with them were routed and the men of the East joined Morgoth. Over the next several decades, Morgoth destroyed the remaining Elven kingdoms, reducing their domain to an island in the Bay of Balar to which many refugees fled, and a small settlement at the Mouths of Sirion under the protection of Ulmo.
Before the Nírnaeth Arnoediad the Man Beren and Elf Lúthien the daughter of Thingol entered Angband and recovered a Silmaril from Morgoth’s crown. It was inherited by their granddaughter Elwing, who joined those dwelling at the Mouths of Sirion. Her husband Eärendil, wearing the Silmaril on his brow, sailed across the sea to Valinor, where he pled with the Valar to liberate Middle-earth from Morgoth.
During the ensuing War of Wrath, Beleriand and much of the north of Middle-earth was destroyed and reshaped. In the end, Morgoth was utterly defeated. His armies were almost entirely slaughtered. The dragons were almost all destroyed, and Thangorodrim was shattered when Eärendil slew the greatest of dragons, Ancalagon the Black, who crashed upon it as he fell. The few remaining dragons were scattered, and the handful of surviving Balrogs hid themselves deep within the earth. Morgoth fled into the deepest pit and begged for pardon, but his feet were cut from under him, his crown was made into a collar, and he was chained once again with Angainor. The Valar exiled him permanently from the world, thrusting him through the Door of Night into the void, and he is not expected to return until the prophesied Dagor Dagorath, when he is expected to meet his final destruction. His evil remained, however, as “Arda Marred," and his will influenced all living creatures.
The Ainu Melkor could initially take any shape, but his first recorded form was "...as a mountain that wades in the sea and has its head above the clouds and is clad in ice and crowned with smoke and fire; and the light of the eyes of Melkor was like a flame that withers with heat and pierces with a deadly cold. At the time he slew the Two Trees and stole the Silmarils, he took to himself the shape of the great Dark Lord: gigantic and terrifying. The diminution of his power in this time and his own desire for lordship destroyed his ability to freely change shape, and he became bound to this one terrible form. His hands were burned by the theft of the Silmarils, and never healed. In his fight with High King Fingolfin, he suffered several wounds; his foot was hewn by Fingolfin's Sword of Ice Crystal, Ringil. At the end of this battle, Thorondor, the great Eagle, swooped down and scarred Morgoth's face with his talons, a wound that also never healed. In battle he wore black armour and wielded Grond, the Hammer of the Underworld. The great battering ram of Mordor was named for this weapon. He also wielded a black spear, and in early texts a poison sword.
Melkor's powers were originally immense – greater than those of any other single Ainu. He shared a part of the powers of every other Vala, but unlike them used it for domination of the whole of Arda. To accomplish this Morgoth dispersed his being throughout Arda, tainting its very fabric; and only Aman was free of it. His person thus became ever more diminished and restricted.
Pity was beyond Morgoth’s understanding, as was courage. As he alone of the Valar bound himself to a physical (and therefore destructible) body, he alone of the Valar knew fear.
Ungoliant, a demon in spider form, helped Melkor destroy the Two Trees. But this alliance was temporary; when Melkor refused to yield the Silmarils to Ungoliant, she attacked him. He had spread his power and malice too thin, and had weakened himself too much to fight back; he escaped only through the arrival of the Balrogs.
When the race of Men awoke, Morgoth (or his servant, depending on the text consulted) temporarily left Angband to live among them. Some men worshipped him, banning Ilúvatar from their hearts.
Morgoth was known to betray his own servants. After the Noldor were defeated, he confined all Men in his service to the lands of Hithlum, their only prize the pillage of that land, though they had fought to win richer lands in Beleriand. Since he could never fully dominate Men, he could never really trust them, and indeed feared them.
In the early versions of Tolkien's stories, Melkor/Morgoth was not seen as the most powerful Ainu. He is described as being equal in power to Manwë, chief of the Valar in Arda. But his power increased in later revisions of the story until he became the most powerful Ainu, and in a late essay more powerful than all of the Valar combined. He developed from a standout among equals into a being so powerful that the other created beings could not utterly defeat him.
Over time, Tolkien altered both the conception of this fallen Ainu and his name. The name given by Fëanor (Morgoth) was present from the first stories. He was for a long time also called Melko. Tolkien vacillated over the Sindarin equivalent of this, which appeared as Belcha, Melegor, and Moeleg. The meaning of the name also varied, related in different times to milka 'greedy' or velka 'flame'. Similarly the 'Old English translations' devised by Tolkien differ in sense: Melko is rendered as Orgel 'Pride' and Morgoth as Sweart-ós 'Black God'. Morgoth is once given a particular 'sphere' of interest: in the early Tale of Turambar Tinwelint (precursor of Thingol) names him "the Vala of Iron".
Much of the text published in The Silmarillion was drawn from earlier, more completely written, drafts of the mythology — and thus reflects the older conception of Morgoth's power; there is less discussion of his marring all of Arda by diluting himself throughout it. In other sections, such as the 1950s draft used for Ainulindalë, the implication of his pervasive power remains clear. While not included in the published 'Silmarillion', other versions of the mythology hold that Melkor will escape the guardianship of Eärendil and return at the end of time. In the final battle, Melkor will be slain by Túrin Turambar with his famous black sword.
In late writings a distinction is made between the Ainu Melkor, the most powerful of Eru’s created beings, and Morgoth, the diminished being that styled itself Dark Lord of Arda. This distinction is not limited to the change in name (‘Arises in Might’ to ‘Dark Enemy’).
As described in Ainulindalë, Melkor’s discord marred the Music of the Ainur in Heaven. Melkor’s thematic variations in that Music amounted to his own self-elaboration. (Each Ainu arose from a divine theme, existing beforehand only in the mind of Eru.) Eä, or the World that Is, is shaped after the Music. Thus, the evil that Melkor weaves into the Music was mirrored in Eä by the evil he wove into the fabric of reality. As a result, the world Arda was "Marred": the conceptions of the Valar never came about, and Melkor's very essence was present in all creation.
Melkor’s inability to perform true creation is tied to the idea that something of his actual being must pass into the things he ‘created’, in order to give them an effective substance and reality. Melkor could not create something as he did not possess the Imperishable Flame; thus he could only create a mockery of those things in Arda. From his Trolls to the Sun (which was made from a flower from a Tree poisoned by Ungoliant, and was thus itself imperfect), Melkor’s power and essence was poured into Arda. Melkor's individual self was diminished as a consequence. He was reduced to Morgoth, the “Dark Enemy” (poetically elaborated as "The Foe of the World").
Morgoth, once the most powerful being in Eä, spent his will on his vast armies and followers, so that in the War of Wrath, as his armies were swept away before the host of Aman, he was captured by Eönwë and cast off his throne. Morgoth's spirit was cast out beyond the Walls of Night, yet his presence remains as the pervasive corruption of the world.