Melkor was once a Vala, and the most powerful of the Ainur. He was full of pride, anger, malice, and a lusted for power and domination. He turned to darkness and brought great ruin upon the earth, marring its originally perfect symmetry. His jealousy and envy of his peers led to great strife amongst the Noldor of Eldamar, causing their revolt and The War of the Great Jewels. He became known as Morgoth (“the Black Foe”) and Bauglir (“the Constrainer”), and he corrupted the hearts of Men with lies and traitorous gifts. When he was finally defeated by the Host of Valinor at the end of the War of Wrath, Melkor was chained, bound, and thrust through the Door of Night and into the Void.
Sauron, who had been one of the Maiar of Aulë, betrayed his kind and became Morgoth's principal lieutenant.
Melkor, like Sauron, also possessed a wide range of other titles, including: 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.
While Aulë, his fellow Ainu, would admit that his creations were simple discoveries made possible by (and therefore “belonging” to) Eru, Melkor’s pride was too great; he aspired to Eru’s level, being the true Creator of all possibilities.
During the Great Music of the Ainur, Melkor attempted to alter the Music by introducing what he believed to be elements of his own design. Through these efforts, he drew many weaker-willed Ainur to him, and they created a counter to Eru’s main theme. However, these alterations did not threaten the Music, but instead elaborated upon Eru’s original intentions: the Music of Eru became deeper and more beautiful because of the strife and sadness created by Melkor’s disharmonies.
Because the Great Music of the Ainur stood as template for all of history and all of material creation in the Middle-earth, there was an aspect of everything in Middle-earth that came of Melkor’s malign influence; everything had now been "corrupted."
Each of the Valar was attracted to a particular aspect of the world that became the focus of his or her powers. Melkor became drawn to terrible extremes and violence — the bitter cold, scorching heat, earthquakes, breakings, utter darkness, burning light, and so on. 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 came into Eä and tipped the balance and drove Melkor out.
Melkor brooded in the darkness at the outer reaches of Arda until one day, Tulkas became distracted for a moment, allowing Melkor the opportunity to re-enter Arda. Once in, he attacked and destroyed the Two Lamps (the only sources of light at the time), plunging Arda into darkness. The island of Almaren, the first home of the Valar on Earth, was destroyed in the violence of the lamps' fall. The Valar withdrew into Aman in the far West, settling in a new country which they fortified heavily and called Valinor. Melkor held dominion over Middle-earth from Utumno, his fortress in the North.
The first reign of Melkor ended after the Elves (the eldest of the Children of Ilúvatar) awoke at the shores of Cuiviénen, and the Valar set out to rescue them from his malice. They waged devastating war on Melkor, destroyed Utumno, and bound Melkor with a specially forged chain, Angainor, and brought him to Valinor, where he was imprisoned in the Halls of Mandos for three ages.
Following his release, Melkor was paroled to Valinor, though many of the Valar were distrustful of him. He pretended to act out of virtue and humility, but in secret plotted harm against the Elves, whose awakening he blamed for his defeat. The Elves most vulnerable to his plots were the Noldor, the most skilled of the three kindreds of Elves that had come to Valinor. When they became aware of his plans, the Valar sent Tulkas to arrest him, only to find that Melkor had already fled. Melkor, with the aid of Ungoliant (a dark spirit in the form of a monstrous spider), destroyed the Two Trees of Valinor, slew Finwë, the King of the Noldor, and stole the three Silmarils (filled with the light of the trees), made by Finwë’s son, Fëanor.
Thereafter, Melkor was known by Morgoth, “Black Foe of the World” (as Fëanor named him), and it was by this name alone that the Eldar knew him.
Morgoth continued his rule in the North of Middle-earth, this time in Angband, a lesser fortress than Utumno, but not as destroyed. He rebuilt it, and raised above it the volcanic triple peak of Thangorodrim. He set the Silmarils into a crown of iron, which he wore at all times.
Fëanor and most of the Noldor pursued him, slaying their kin, the Teleri, along the way, and incurring the Doom of Mandos. When they arrived in Beleriand, the region of Middle-earth nearest Angband, the Noldor established kingdoms and waged war on Morgoth. Soon afterwards, the Sun and the Moon arose for the first time, and Men, if they had not done so already, awoke.
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 who had allied with them were routed and the men of the East joined Morgoth. Prior to the Nírnaeth Arnoediad, the Man Beren and the Elf Lúthien (daughter of Thingol) had entered Angband and recovered a Silmaril from Morgoth’s crown.
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.
During the ensuing War of Wrath, Beleriand and much of the north of Middle-earth was destroyed and reshaped. Morgoth was utterly defeated in the end, his armies almost entirely slaughtered. Almost all of the dragons were destroyed, and Thangorodrim was shattered when Eärendil slew Ancalagon the Black, the greatest of dragons, who crashed upon it as he fell. The few remaining dragons were dispersed, 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 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, excluded from Arda until the prophesied Dagor Dagorath, when he would meet his final destruction. His evil remained, however, as "Arda Marred," and his will influenced all living creatures.
Melkor is a compound of mbelek- (melek, "great, mighty, powerful"; root BEL, MBEL) + óre.
The older form of Melkor is said to be Melkórë.
The name Morgoth is Sindarin and means “Black Foe of the World.”
The Sindarin equivalent of Melkor was Belegûr, but it was never used; instead a deliberately resembling name Belegurth, meaning "Great Death,” was used.