Morgoth Bauglir ([ˈmɔrɡɔθ ˈbau̯ɡlir]; originally Melkor [ˈmɛlkor]) is a character, one of the godlike Valar and the primary antagonist of Tolkien's legendarium, the mythic epic published in parts as The Silmarillion, The Children of Húrin, Beren and Lúthien, and The Fall of Gondolin.

Melkor is the most powerful of the Valar but he turns to darkness and is renamed Morgoth, the primary antagonist of Arda. All evil in the world of Middle-earth ultimately stems from him. One of the Maiar of Aulë betrays his kind and becomes Morgoth's principal lieutenant and successor, Sauron.

Melkor has been interpreted as analogous to Satan, once the greatest of all God's angels, Lucifer, but fallen through pride; he rebels against his creator. Morgoth has likewise been likened to John Milton's fallen angel in Paradise Lost, again a Satan-figure. Tom Shippey has written that The Silmarillion maps the Book of Genesis with its creation and its fall, even Melkor having begun with good intentions. Marjorie Burns has commented that Tolkien used the Norse god Odin to create aspects of several characters, the wizard Gandalf getting some of his good characteristics, while Morgoth gets his destructiveness, malevolence, and deceit. Verlyn Flieger writes that the central temptation is the desire to possess, something that ironically afflicts two of the greatest figures in the legendarium, Melkor and Fëanor.

The name Morgoth is Sindarin (one of Tolkien's invented languages) and means "Dark Enemy" or "Black Foe". Bauglir is also Sindarin, meaning "Tyrant" or "Oppressor". "Morgoth Bauglir" is thus an epithet. His name in Ainulindalë (the creation myth of Middle-earth and first section of The Silmarillion) is Melkor, which means "He Who Arises in Might" in Quenya. This too is an epithet, since he, like all the Valar, had another true name in Valarin (in the legendarium, the language of the Valar before the beginning of Time), but this name is not recorded. The Sindarin equivalent of Melkor is Belegûr, but it is never used; instead, a deliberately similar name, Belegurth, meaning "Great Death", is employed. Another form of his name is Melko, simply meaning "Mighty One".

Like Sauron, he has a host of other titles: Lord of the Dark, the Dark Power of the North, the Black Hand, and Great Enemy. The Edain, the Men of Númenor, call him the Dark King and the Dark Power; the Númenóreans corrupted by Sauron call him the Lord of All and the Giver of Freedom. He is called "Master of Lies" by one of the Edain, Amlach.

Melkor is renamed "Morgoth" when he destroys the Two Trees of Valinor, murders Finwë, the High King of the Noldor Elves, and steals the Silmarils in the First Age.

Before the creation of Eä and Arda (The Universe and the World), Melkor is the most powerful of the Ainur, the "angelic beings" created by Eru Ilúvatar. Melkor, dissatisfied that Eru had abandoned the Void, seeks to emulate his creator and fill the Void with sentient beings. This, however, requires the Flame Imperishable, the Secret Fire, which belongs to Eru alone; Melkor cannot find it. He contends with Eru in the Music of the Ainur, introducing themes of his own. He draws many weaker-willed Ainur to him. Ironically, these attempts do not truly subvert the Music, but elaborate Eru's original intentions: the Music of Eru takes on depth and beauty through the strife and sadness Melkor's disharmonies introduce. Unlike Aulë, Melkor is too proud to admit that his creations are made possible entirely by Eru. Instead, Melkor aspires to rival Eru.

In an early draft, Tolkien has the Elf Finrod state that "there is nothing more powerful that is conceivable than Melkor, save Eru only". In The Silmarillion, Eru Ilúvatar similarly states that "Mighty are the Ainur, and mightiest among them is Melkor".

Since the Great Music stands as template for all of material creation, the chaos introduced by Melkor's disharmonies is responsible for all evil. Everything in Middle-earth is tainted by his influence. In Morgoth's Ring, Tolkien draws an analogy between the One Ring, into which Sauron commits much of his power, and all of Arda – "Morgoth's Ring" – which contains and is corrupted by Melkor's power until the Remaking of the World. The Valaquenta tells how Melkor seduced many of the minor Ainur, the Maiar, into his service.

Tolkien's comparison of "Morgoth's Ring" with Sauron's One Ring

	Sauron	Morgoth
Action	Put much of his power into the One Ring	"Melkor 'incarnated' himself (as Morgoth) permanently"; Transmuted "the greater part of his original 'angelic' powers, of mind and spirit"
Purpose	Gain power over the other Great Rings, and over the physical world	Gain "a terrible grip upon the physical world", the ability "to control the hroa, the 'flesh' or physical matter, of Arda."
Effect	Power "concentrated" in the One Ring	Power "disseminated" through "the whole of Middle-earth"
Outcome	Utterly eradicated when the One Ring is destroyed	Eradication not possible; probable "irretrievable ruin of Arda" by fighting Morgoth

After the Creation, many Ainur enter into Eä. The most powerful of them are the Valar, the Powers of the World; the lesser, the Maiar, act as their followers and assistants. They set about the ordering of the universe and Arda within it, as they understand the themes of Eru. Melkor and his followers enter Eä as well, but he is frustrated that his colleagues do not recognize him as leader of the new realm, despite his great knowledge. In anger and shame, Melkor sets about ruining and undoing whatever the others do.

Each of the Valar is attracted to a particular aspect of the world. Melkor is drawn to extremes and violence—bitter cold, scorching heat, earthquakes, darkness, burning light. His power is so great that at first the Valar cannot restrain him; he contends with their collective might. Arda is unstable until the Vala Tulkas enters Eä and tips the balance.

Driven out by Tulkas, Melkor broods in darkness, until Tulkas is distracted. Melkor destroys the Two Lamps and the Valar's land of Almaren. Arda is plunged into darkness and fire, and Melkor withdraws to Middle-earth. In later versions, Melkor also disperses agents throughout Arda, digging deep into the earth and constructing great pits and fortresses, as Arda is marred by darkness and rivers of fire.

The Valar withdraw into Aman in the far West. The country where they settle is called Valinor, which they heavily fortify. Melkor holds dominion over Middle-earth from his fortress of Utumno in the North. His first reign ends after the Elves, the eldest of the Children of Ilúvatar, awake at the shores of Cuiviénen, and the Valar resolve to rescue them from his malice. Melkor captures some Elves before the Valar attack. He tortures and corrupts them, breeding the first Orcs. Other versions describe Orcs as corruptions of Men, or as soulless beings animated solely by the will of their evil lord. His fortress Utumno disperses deathly cold and brings on an endless winter in the North; for the sake of the Elves, the Valar wage a seven-year war with Melkor, defeating him after laying siege to Utumno. These battles further mar Arda. Tulkas defeats Melkor, binds him with a specially forged chain, Angainor, and brings him to Valinor. He is imprisoned in the Halls of Mandos for three ages.

Upon his release, Melkor is paroled to Valinor, though a few of the Valar continue to mistrust him. He pretends humility and virtue, but secretly plots harm toward the Elves, whose awakening he blames for his defeat. The Noldor, most skilled of the three kindreds of Elves that had come to Valinor, are most vulnerable to his plots, since they are eager for his knowledge. While instructing them, he awakens unrest and discontent among them. When the Valar become aware of this, they send Tulkas to arrest him, but Melkor has already fled. With the aid of Ungoliant, a dark spirit in the form of a monstrous spider, he destroys the Two Trees of Valinor, kills Finwë, the King of the Noldor, and steals the three Silmarils, jewels made by Finwë's son Fëanor, filled with the light of the Trees. Fëanor thereupon names him Morgoth, "Black Foe", and the Elves know him by this name alone afterwards.

Morgoth resumes his rule in the North of Middle-earth, this time in the half-ruined fortress of Angband. He rebuilds it, and raises above it the volcanic triple peak of Thangorodrim. The Silmarils he sets into a crown of iron, which he wears at all times. Fëanor and most of the Noldor pursue him, along the way slaying their kin the Teleri and incurring the Doom of Mandos. On arriving in Beleriand, the Noldor establish kingdoms and make war on Morgoth. Soon, the Sun and the Moon rise for the first time, and Men awake. The major battles include 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 Siege of Angband is broken, and the battle of Nírnaeth Arnoediad (Unnumbered Tears) when the armies of the Noldor and the Men allied with them are routed and the men of the East join Morgoth. Over the next several decades, Morgoth destroys the remaining Elven kingdoms, reducing their domain to an island of refugees in the Bay of Balar, and a small settlement at the Mouths of Sirion under the protection of Ulmo.

Before the Nírnaeth Arnoediad, the Man Beren and the Elf Lúthien enter Angband and recover a Silmaril from Morgoth's crown after Luthien's singing sends him to sleep. It is inherited by their granddaughter Elwing, who joins those dwelling at the Mouths of Sirion. Her husband Eärendil, wearing the Silmaril on his brow, sails across the sea to Valinor, where he pleads with the Valar to liberate Middle-earth from Morgoth.

During the ensuing War of Wrath, Beleriand is destroyed. Morgoth summons many Men to his side during the fifty-year conflict, the longest and bloodiest in Arda's history. Morgoth is utterly defeated. Thangorodrim is shattered when Eärendil kills the greatest of dragons, Ancalagon the Black, who crashes upon it as he falls. The few remaining dragons are scattered, and the few surviving Balrogs hide themselves deep within the earth. Morgoth flees into the deepest pit and begs for pardon, but his feet are cut from under him, his crown is made into a collar, and he is chained once again with Angainor. The Valar exile him permanently from the world, thrusting him through the Door of Night into the void until the prophesied Dagor Dagorath, when he will meet his final destruction. But his evil remains, and his will influences all living creatures.

In this more complete version of a story summarized in Quenta Silmarillion, Húrin and his younger brother Huor are leaders of the House of Hador, one of the three kindred of elf-friends. At Nírnaeth Arnoediad they cover the escape of Turgon to Gondolin by sacrificing their army and themselves. Huor is slain, but Húrin is brought before Morgoth alive. As revenge for his aid to Turgon and his defiance, Morgoth curses Húrin and his children, binding Húrin to a seat upon Thangorodrim and forcing him to witness all that happens (using Morgoth's long sight) to his children in the succeeding years. The encounter with Húrin, is set out in more detail than in The Silmarillion, and in a more connected narrative than in Unfinished Tales. It gives the first allusion to the corruption of Men by Morgoth soon after their awakening, and the assertion by Morgoth of his power over the entire Earth through "the shadow of my purpose".

Melkor is mentioned briefly in the chapter "A Knife in the Dark" in The Lord of the Rings, where Aragorn sings the story of Tinúviel and briefly recounts the role of Morgoth ("the Great Enemy") in the wider history of the Silmarils.

In the early versions of Tolkien's stories, Melkor/Morgoth is not seen as the most powerful of the Valar. He is described as being equal in power to Manwë, chief of the Valar in Arda. But his power increases in later revisions of the story until he becomes the most powerful among them, and in a late essay more powerful than all of the other Valar combined. He develops 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 character 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".