I am the Elder King: Melkor, first and mightiest of all the Valar, who was before the world and made it. The shadow of my purpose lies upon Arda, and all that is in it bends slowly and surely to my will.
~ Morgoth to Hurin, The Words of Hurin and Morgoth.
Morgoth, originally named Melkor, is the primary villain of the Arda universe of J.R.R. Tolkien, which is mainly focused on Middle-earth, and ultimately, the main antagonist of the Middle-Earth legendarium. He is the primary antagonist of the mythopoeic novel The Silmarillion, the secondary antagonist and Bigger Bad of The Children of Húrin, and the in-turn eponymous primary antagonist of The History of Middle-Earth. He is an evil Vala and the first Dark Lord, and the master and predecessor of Sauron. Morgoth is also the de-facto ultimate evil of the whole Earth, known as Arda.
And he descended upon Arda in power and majesty greater than any other of the Valar, as a mountain that invades 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.
~ The Silmarillion on Morgoth's coming to Arda.
When he first came to Arda, Melkor took the form of a gigantic, ice-white humanoid being, filled with flames. This form was terrifying; it was the size of a giant mountain, with his head just touching the uppermost clouds. This projected his arrogance, greed for power, and hatred of his Valarian siblings on the early Arda, and it severely damaged the world with its mere presence alone.
The more time Morgoth spent in the world, the more his dark leaked into it, and corrupted what his Father Eru had originally designed. As time went on, he became manifested as the Dark Lord when his shapeshifting abilities were stripped from him from all of the evil he was committing. As the Dark Lord, he was superhumanly tall, approximately 500 feet, "like a tower", and covered in dark storm clouds and a dark powerful aura which humbled all who saw him. He wore an Iron Crown at all times, as his means of others identifying him with kingship of the world, and he placed the Silmarils in his crown when he had stolen them.
....hatred devoured him, and in the domination of his servants and the inspiring of them with lust of evil he spent his spirit.
~ The Silmarillion (on Morgoth).
Morgoth was a very standoffish, intimidating, and unapproachable opponent, because of his being in the presence of Ilúvatar had given him almost godlike powers. However, he was a skilled, influential, and manipulative politician and could charm his enemies, as he tried to do with Hurin when he captured him.
When seen as Melkor, he was perceived as resplendent, majestic, and noble, even by his enemies, but when he became Morgoth, he became a giant looming threat which was feared by even the noblest warriors on Earth. Literally only five people on Earth ever challenged him: Fingolfin, Beren and Luthien, Hurin, and also Earendil. This clearly shows his formidable and authoritative might, with Fingolfin being the only one to ever enter into a duel with him. Although Morgoth won the duel, Fingolfin gave him seven wounds, which clearly shows Morgoth's power, (albeit diminished due to his evils) because if a skilled Elf swordsman could only give Morgoth seven wounds then he was clearly impressive, resilient, powerful, and durable.
Unlike the other Valar, Morgoth knew fear and pain. This was most likely a punishment given to him because of his heinous and iniquitous crimes. He could feel the pain of the Silmarils, and also the wounds given him by Thorondor the eagle, and Fingolfin scarred him forever and caused him a burning pain. Luthien commented that Morgoth's life was a "burden", and she rid him of this "burden" for awhile by putting him into sleep.
Morgoth was also very domineering and arrogant, as he loved challenging people and reducing their wills to dust. He prided himself on his immense powers, giant size, and intelligence, and knew that he would most likely come off better in any duel. However, these tables got turned in his epic downfall.
Morgoth may rank as one of (if not the) most evil, terrifying, and powerful antagonist in fiction, because he was almost successful in his goals, and it literally took everyone's will to defeat him.
Yet I may come at you, and all your accursed house; and you shall be broken on my will, though you all were made of steel.
~ Morgoth to Hurin.
But upon all whom you love my thought shall weigh as a cloud of Doom, and it shall bring them down into darkness and despair. Wherever they go, evil shall arise. Whenever they speak, their words shall bring ill counsel. Whatsoever they do shall turn against them. They shall die without hope, cursing both life and death.
~ Morgoth to Hurin.
Behold! The shadow of my thought shall lie upon them wherever they go, and my hate shall pursue them to the ends of the world.
Fool, little among Men, and they are the least of all that speak! Have you seen the Valar, or measured the power of Manwë and Varda? Do you know the reach of their thought? Or do you think, perhaps, that their thought is upon you, and that they may shield you from afar?
~ Morgoth to Hurin about the Valar.
What wouldst thou have more? Dost thou desire all the world for thy belly? I did not vow to give thee that. I am its Lord.
~ Morgoth to Ungoliant.
Nay! Thou hast thy due. For with my power that I put into thee thy work was accomplished. I need thee no more. These things thou shall not have, nor see. I name them unto myself forever.
~ Morgoth refuses to give Ungoliant the Silmarils.
Allusions in Other Works
Morgoth appears as a character in Legendary Frog's parody series of flash videos, One Ring to Rule them All. Ironically, he only appears in the third video.
In the 3rd video, Melkor acts as Sauron's old master (or college roommate), and gives him a visit. However, he steals his spices and seasonings for Legolas.
How Morgoth created the Orcs is up for much debate, because Tolkien himself says "Who has explored the mind of Morgoth?" This being said, the creation of Orcs is entirely open to speculation. There is a disturbing theory that, as Orcs are bow-legged, Morgoth smashed their legs in repeatedly, thus breaking them, and over time their descendants "healed" to being a bow-legged creature. Also, their elongated arms could be the result of Morgoth gruesomely stretching them on racks or from his hanging them from chains on high vaulted subterranean halls. And finally, their charred black skin could simply be the result of his burning them alive through intense flame.
While not appearing in the film series (most likely because he no longer existed in the world at the time of the said series because of his banishment to the Void in the War of Wrath in The Silmarillion), Morgoth's most powerful servants such as Sauron still thrive, and Morgoth was mentioned in The Lord of the Rings: The Two Towers as when Gandalf speaks of his battle with the Balrog, he refers to it as "The Balrog of Morgoth". He was also mentioned in The Hobbit: The Battle of the Five Armies when Galadriel calls Sauron "Servant of Morgoth".
Ironically, Morgoth was not even mentioned in the novel of The Hobbit, because by that time Tolkien had not yet conceived his mighty mythology, and he had only begun to create the barest aspects.
Morgoth appears in his earliest incarnation in the novels Unfinished Tales which compare the earliest scripture of The Lord of the Rings with the final version. In this version, Morgoth is named Melko instead of Melkor, and is either another name for him or an epithet of Melkor itself.
Sauron is said by Tolkien to have been certainly more successful than Morgoth, given that Sauron was far more cunning, and possessed shape shifting and deception to fool others. Morgoth is compared to Sauron by Tolkien because the whole Earth was "Morgoth's Ring". However, Morgoth is much more evil, powerful, and majestic, than Sauron, and is not to be overlooked or taken lightly.
It might be a bad thing if Dagor Dagorath happens because Morgoth is supposed to die in the prophecy, and if he dies, the balance between good and evil will collapse because he is the ultimate evil in Middle-earth. However, it is possible that Morgoth may be revived somehow and imprisoned once again when the prophecy is fulfilled, this time for good.