Melkor was the most powerful of the Ainur, but turning to darkness, became Morgoth Bauglir, the ultimate antagonist of Arda, from whom all evil in the world of Middle-earth ultimately stems. Sauron, one of the Maiar of Aulë, betrayed his kind and became Morgoth's principal lieutenant.
Morgoth influence lingered 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 and means "Black Foe of the World"; Bauglir is also Sindarin, meaning "Tyrant" or "Oppressor". "Morgoth Bauglir" is actually an epithet: his name in Ainulindalë is Melkor, which means 'He Who Arises In Might' in Quenya. This too is an epithet since he, like all the Ainur, had another true name in Valarin. 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.
Melkor was not called "Morgoth" until he destroyed the Two Trees, murdered Finwë and stole the Silmarils in the First Age. The darker name was then bestowed by Fëanor, son of Finwë; and the Elves called him thereafter by that name alone.
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, Melkor was the most powerful of the Ainur. He contended with Eru, 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.
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 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, there was an aspect of everything in Middle-earth that came of Melkor’s malign influence; everything had been "corrupted".
Because Morgoth was the most powerful creature in Arda, many "flocked to his banner." Morgoth's chief servants were Maiar he corrupted or monsters he created: Sauron, later the Dark Lord of Mordor and his chief servant; the Balrogs, including Gothmog, the Lord of Balrogs and High-Captain of Angband; Glaurung, the Father of Dragons; Ancalagon "the Black", greatest of the Winged Dragons; Carcharoth, the mightiest wolf that ever lived; Draugluin, Sire of Werewolves; and Thuringwethil, Sauron's vampire messenger.
Ungoliant, a demon in spider form, helped Melkor destroy the Two Trees. This alliance was temporary, however; 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 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.
