After Morgoth was released, he stole the Silmarils. The Great Spider Ungoliant tried to take them from him, but Morgoth gave a terrible cry and the Balrogs heard him and came to his aid. They freed Morgoth from Ungoliant's webs with their flaming whips and Ungoliant fled from them in fear.
The Elf-craftsman Feanor marched on Angband to retrieve the Silmarils he had made, and Balrogs came forth and surrounded him. Feanor fought long and hard and received many wounds from their fiery whips until at last he was struck down by Gothmog, the Lord of the Balrogs. Feanor later died of his wounds. Afterwards, Morgoth said he would negotiate with the Elves, but he sent Balrogs with his emissaries and Feanor's son Maedhros was taken captive.
During the Battle of Sudden Flame in the year 455 of the First Age, Glaurung the Dragon led a host of Balrogs and Orcs to attack the Elves and Men. At the Battle of Unnumbered Tears in 472, Gothmog, Lord of the Balrogs, slew Fingon, the High King of the Noldor. Another Balrog trapped Fingon with its fiery whip while Gothmog cleaved Fingon's helmet with his axe, and then the Balrogs beat him into the ground with their maces.
Balrogs were among the forces that attacked the hidden realm of Gondolin in 510. Gothmog battled the Elf-lord Ecthelion and they both were slain. Another Balrog tried to prevent the escape of Tuor, Idril, and Earendil in the pass called the Eagles' Cleft. Glorfindel engaged the Balrog in battle to allow the others to escape. They fought on the precipice and both fell into the abyss to their deaths.
When the Valar vanquished Morgoth in the War of Wrath at the end of the First Age, most of the Balrogs were destroyed, but it is said that a few fled into deep underground caverns. The fate of only one of these is known: It went to the Misty Mountains and hid at the roots of the Redhorn, where it lurked for over 6,000 years.
In 1980 of the Third Age, the Dwarves of Khazad-dum were delving under the Redhorn for mithril when they encountered the Balrog. The Balrog may have been roused by their mining or it may already have awakened in response to the growing power of Sauron. King Durin VI was slain by the Balrog, and the next year it killed Durin's son Nain I. The Dwarves abandoned Khazad-dum and it became known as Moria, the Black Chasm, and Orcs and other evil creatures came to dwell there.
The Fellowship of the Ring entered Moria on January 13, 3019, and the Balrog became aware of their presence. On January 15, the Balrog entered the Chamber of Mazarbul after the Fellowship had left and it perceived Gandalf beyond the door. Like the Balrog, Gandalf too was of the order of Maiar. Gandalf placed a shutting spell on the door, but the Balrog cast a counter-spell and was nearly able to open the door, and then Gandalf spoke a word of Command. In the struggle between the two, the door shattered and the roof of the chamber collapsed.
The Balrog escaped and pursued the Fellowship to the Bridge of Khazad-dum. Gandalf remained on the bridge and denied the Balrog passage. The Balrog wielded a whip of many thongs and a flaming sword. Gandalf used Glamdring to shatter the blade of the Balrog's sword into molten fragments. He then struck the bridge with his staff and the stone cracked under the Balrog's feet. The Balrog fell into the abyss with a terrible cry and it lashed its whip and caught Gandalf around the knees, dragging the Wizard in after it.
The Balrog and Gandalf fell together for a long time, and Gandalf was burned by the creature's fire. They landed in a subterranean lake at the root of the mountains. The Balrog's fire was quenched and it became slimy, but it remained strong and it continued to battle with Gandalf. Then the Balrog fled through the tunnels to the Endless Stair and climbed to the peak of the Silvertine with Gandalf in pursuit.
The Balrog burst into flame again, and it fought Gandalf in the Battle of the Peak from January 23 to 25. At last Gandalf threw the Balrog down from the peak and it shattered the mountain-side and died. Gandalf died as well, but he was returned to life as Gandalf the White.
There are no further reports of Balrogs in the history of Middle-earth, so the Balrog of Moria may well have been the last of its kind.
Do Balrogs have wings?
This is one of the most debated issues among Tolkien fans. There is no definitive answer. The question centers mainly on these sentences describing the Balrog:
The Balrog ... halted again, facing him, and the shadow about it reached out like two vast wings ... It stepped forward slowly on to the bridge, and suddenly it drew itself up to a great height, and its wings were spread from wall to wall ...
The Fellowship of the Ring: "The Bridge of Khazad-dum," p. 344-45
This passage can be read two ways. Those who believe that Balrogs have wings interpret this as saying that the Balrog had physical wings on its body that it spread from wall to wall. Those on the opposing side say that the wings that were spread out were not real wings, but were instead the shadow around the Balrog, which is described as reaching out like wings.
There are a few other ambiguous passages that are relevant to the debate:
"Thus they roused from sleep a thing of terror that, flying from Thangorodrim, had lain hidden ..."
Appendix A: "Durin's Folk," p. 353
Here flying may be used in the sense of traveling through the air, or it may mean fleeing.
"Swiftly they [the Balrogs] arose, and they passed with winged speed over Hithlum ..."
The History of Middle-earth, vol. X, Morgoth's Ring: "Of the Thieves' Quarrel," p. 297
The phrase "winged speed" may be used literally or metaphorically.
"Too much it reminded me of the shadow in Moria - the shadow of the Balrog."
The Fellowship of the Ring: "The Great River," p. 404
In this quote, Gimli is reacting to encountering a Winged Nazgul for the first time. Gimli may be referring to the physical shape of the creature, or the shape of its shadow, or even simply the feeling of terror it evoked.
Another issue cited by those who think Balrogs do not have wings it that Balrogs do not fly in several instances when it would benefit them to do so. For example, when Glorfindel battled the Balrog at Gondolin, the Balrog fell to its death and did not save itself by flying as one might expect if it had wings. The Balrog that Gandalf battled also fell into an abyss and was later cast down from the peak of the Silvertine.
Those who believe that Balrogs have wings explain this lack of flying in a variety of ways. Some say that Balrogs could not use their wings to fly. Others say that in these particular circumstances the Balrogs were unable to fly, either because of the trajectory of the fall, the narrowness of the space, or the injuries sustained in combat.
In the end, there is nothing in Tolkien's writings that definitively states whether he intended for Balrogs to have wings or not. So - unless someone encounters a Balrog in the modern world and lives to tell the tale - the question of whether Balrogs have wings remains open to each reader's interpretation.
For a more detailed look at this issue, see Do Balrogs have wings, and can they fly? by Conrad Dunkerson at the Tolkien Meta-FAQ.
Other Names:
Also called Valarauko (singular) and Valaraukar (plural) in Quenya.
The Balrog of Moria was called Durin's Bane because it killed King Durin VI. A bane is a cause of death, ruin or harm. Gandalf called it Flame of Udun - which was another name for Morgoth's stronghold Utumno where the Balrogs once dwelled. Aragorn referred to it as the Terror and Legolas called it an elf-bane because in the First Age a number of Elves were slain by Balrogs.
Etymology:
The Sindarin name Balrog means "Demon of Might" from bal meaning "power" and rog meaning "demon."
The Quenya name Valarauko also means "Demon of Might" from val meaning "power" and rauko meaning "demon."