According to The Silmarillion, the Valaraukar (which were called Balrogs in Middle-Earth) were Maiar (angelic beings) that were "scourges of fire". They were seduced by the evil Vala Melkor, who corrupted them to his service in the days of his splendor before the making of Arda.
Upon the waking of the Elves, the Valar captured Melkor and destroyed his fortresses Utumno and Angband. But they overlooked the deepest pits, where, with many of Melkor's other allies, the Balrogs fled into hiding. When Melkor returned to Middle-earth from Valinor, now bearing the epithet Morgoth, he was attacked by Ungoliant, a spider-like creature; his piercing scream drew the Balrogs out of hiding to his rescue.
When the Noldor arrived in Beleriand in pursuit of Morgoth, they won a swift victory over his Orcs in the Dagor-nuin-Giliath. Fëanor pressed on towards Angband; but the Balrogs came against him, and Fëanor was mortally wounded by Gothmog, Lord of Balrogs. Fëanor's sons fought off the Balrogs, but Fëanor died of his wounds shortly afterward.
In The Lays of Beleriand, The Lay of Leithian mentions Balrog captains leading Orcs: "the Orcs went forth to rape and war, and Balrog captains marched before".
Tolkien tells of two Balrogs slain by Elves in the fall of Gondolin. During the assault on the city, Ecthelion of the Fountain fought Gothmog, and "each slew the other." Glorfindel fought a Balrog who waylaid an escape party from the fallen city; both fell from the mountainside in the struggle and perished.
In the War of Wrath that ended the First Age, most of the Balrogs were destroyed, although at least one, the Balrog known as Durin's Bane, managed to escape and hide in "caverns at the roots of the earth".
In The Fellowship of the Ring, the Fellowship ventured through Moria and were attacked in the Chamber of Mazarbul by Orcs and the Balrog. Gandalf faced the Balrog at the Bridge of Khazad-dûm and broke the Bridge, but was dragged down by the Balrog. He slew the Balrog but perished himself at the same time — to be sent back as the more powerful Gandalf the White.
Gothmog at the Storming of Gondolin illustration by Tom LobackGothmog appears in various versions of material. He is physically massive and strong, and in one version he is some 12 feet tall. He wields a black axe and whip of flame as his weapons.
He holds the titles of the Lord of the Balrogs (but see Lungorthin below), the High Captain of Angband, and Marshal of the Hosts.
In the Second Battle, Dagor-nuin-Giliath, he leads a force that ambushes Fëanor and wounds him mortally. He leads Balrogs, Orc-hosts, and Dragons as Morgoth's commander in the field in the Fifth Battle, Nírnaeth Arnoediad, and slays Fingon, High King of the Noldor. In that same battle, he captures Húrin of Dor-lómin, who had slain his personal guard of Battle-trolls, and brings him to Angband. As Marshal of the Hosts he is in command of the Storming of Gondolin. He is about to kill Tuor when Ecthelion of the Fountain, a Noldorin Elf-lord, intervenes. Gothmog fights Ecthelion in single combat, and they kill each other.
In The Book of Lost Tales, Tolkien describes Kosomot, the original version of Gothmog, as a son of Morgoth and the ogress Fluithuin or Ulbandi.
Gothmog is Sindarin and means 'Dread Oppressor'.
Kosomot is often considered Gothmog's Quenya name; however, in the Quenya name-list of The Fall of Gondolin another version appears, Kosomoko.
In The Lord of the Rings, a different character bears the name "Gothmog"; see Gothmog (Third Age).
Lungorthin appears in Tolkien's early Lay of the Children of Húrin as "Lungorthin, Lord of Balrogs". This might be another name for Gothmog (above), though Christopher Tolkien thought it more likely that Lungorthin was simply "a Balrog lord".
The Balrog that was found in Moria, as interpreted in a painting by Markus RönckeThis Balrog appears in The Lord of the Rings, encountered by the Fellowship of the Ring in the Mines of Moria.
It survived the defeat of Morgoth in the War of Wrath and escaped to hide beneath the Misty Mountains. For more than five millennia, the Balrog remained in its deep hiding place at the roots of the mountains in Khazad-dûm, until in the Third Age the mithril-miners of Dwarf-King Durin VI disturbed it (or released it from its prison) in T.A. 1980. Durin was killed by the Balrog, whence it was called Durin's Bane by the Dwarves.
The Dwarves attempted to fight the Balrog, but its power was far too great. Despite their efforts to hold Khazad-dûm against it, King Náin and many other Dwarves were killed and the survivors were forced to flee. This disaster also reached the Silvan Elves of Lórien, many of whom also fled the "Nameless Terror" (it was not recognized as a Balrog at the time). The Elves called the place Moria, the "Black Pit" or "Black Chasm"(though the name Moria also appears on the West Gate of Moria, constructed thousands of years earlier in the Second Age).
For another 500 years, Moria was left to the Balrog; though according to Unfinished Tales, Orcs crept in almost immediately after the Dwarves were driven out, leading to Nimrodel's flight. Around T.A. 2480 Sauron began to put his plans for war into effect, and he sent Orcs and Trolls to the Misty Mountains to bar all of the passes: Some of these creatures came to Moria, and the Balrog allowed them to remain.
During the reign of Thráin II, the Dwarves attempted to retake Moria in the War of the Dwarves and Orcs, culminating in the Battle of Azanulbizar before the eastern gate of Moria in T.A. 2799. This was a victory for the Dwarves, but the presence of the Balrog prevented their occupying Moria. Dáin Ironfoot, having slain the Orc Azog near the gate, perceived the terror of the Balrog within and warned Thráin that Moria was unachievable until some force could change the world and remove the Balrog. The Dwarves thus departed and resumed their exile. Despite Dáin's warning, Balin attempted to recolonize Moria in T.A. 2989, but his party was destroyed.
In January T.A. 3019, the Fellowship of the Ring travelled through Moria on the way to Mount Doom. They were attacked in the Chamber of Mazarbul by Orcs. The Fellowship fled through a side door, but when the wizard Gandalf the Grey tried to place a "shutting spell" on the door to block the pursuit behind them, the Balrog entered the chamber on the other side and cast a counterspell. Gandalf spoke a word of Command to stay the door, but the door shattered and the chamber collapsed. Gandalf was severely weakened by this encounter. The company fled with him, but the Orcs and the Balrog, taking a different route, caught up with them at the bridge of Khazad-dûm. The Elf Legolas instantly recognized the Balrog and Gandalf tried to hold the bridge against it. As Gandalf faced the Balrog he proclaimed, "You cannot pass!", and broke the bridge beneath the Balrog. As it fell, the Balrog wrapped its whip about Gandalf's knees, dragging him to the brink. As the Fellowship looked on in horror, Gandalf cried "Fly, you fools!" and fell.
After the long fall, the two landed in a subterranean lake, which extinguished the flames of the Balrog's body; however it remained "a thing of slime, stronger than a strangling snake". They fought in the water, with the Balrog clutching at Gandalf to strangle him, and Gandalf hewing the Balrog with his sword, until finally the Balrog fled into ancient tunnels of unknown origin. Gandalf pursued the creature for eight days, until they climbed to the peak of Zirakzigil, where the Balrog was forced to turn and fight once again, its body erupting into new flame. Here they fought for two days and nights. In the end, the Balrog was defeated and cast down, breaking the mountainside where it fell "in ruin". Gandalf himself died following this ordeal, but he was later sent back to Middle-earth with even greater powers, as Gandalf the White, "until his task was finished".