Melkor’s atrocities for thousands of years in Arda were infinite and so profound that the world was poisoned forever.
And he not only sullied the earth’s bowels but perverted all kinds of creatures and spirits in Arda and Ëa. It was precisely that what he did with some Ainur long before the creation of Middle-earth.
The name Balrog comes from Sindarin (the dialect of the Sindar Elves that became the one spoken by the Elves in Middle-earth) and means “Demon of Might”. They also had a name in Quenya (the ancient dialect of the Elves of Valinor), Valaraukar, which means “Demon of Power”.
They were spirits born from the thought of Eru-Ilúvatar, like Melkor but much less powerful (the so-called Maiar) and who fell under his dominion, attracted by his darkness. Along with him, they became Balrogs, the most faithful servants of the Dark Lord, except for Sauron.
In the first versions of The Silmarillion, Tolkien wrote that there were hundreds of Balrogs, being less fearsome and more abatable even by mere mortals. However, in the final version, it was finally made official that there were an undetermined number of no more than 7 and no less than 3.
Although we will never know it, this change was perhaps motivated by a great intention: to obtain that the figure of Melkor through the Balrogs was even more imposing. Converting the Balrogs into beings of terrible and scarce power, it made that the fact that they were to the service of Melkor turned this one into a being of supreme power.
As for their appearance, they were humanoid beings of colossal size. Around them, they emitted a mixture of fire and shadow, which hid their authentic appearance and only hinted at that figure standing on two legs and with two powerful arms, whose hands (or paws) held a flaming sword and a whip. Both weapons looked freshly melted because of the extreme heat emanating from their bodies, the Balrogs.
Undoubtedly their appearance was aimed at creating terror to the adversaries with their mere presence, although being Ainur, perhaps they could change their shape, although it is never specified. It is also unclear if they have wings since the shadows moved, creating effects that made us think that they possessed them, but that is something true.
As fallen Ainur (Maiar, more specifically) in the service of Melkor, they arrived in Arda with their master in the Years of the Lamps, when the world was young and paradisiacal. After Morgoth’s imprisonment in Aman, the Balrogs escaped from the Valar and hid in Angband to hibernate during most Years of the Trees.
When Melkor escaped with Ungoliant, returning to the vicinity of Angband after destroying the Two Trees, the Balrogs awoke from their lethargy to hear the cries of their master fighting with the giant spider.
They all came and were able to win Ungoliant, who fled. The Balrogs were going to pursue her as they intended to destroy her, but Melkor urged his servants to return to the fortress of Angband to begin new plans of domination.
With the arrival of the Noldor to Beleriand at the beginning of the First Age, the Balrogs became the greatest enemies of the Elves, being their most fearsome executioners during the numerous battles that took place throughout several centuries.
The Balrogs, specifically their chief Gothmog, killed Fëanor when he went alone to the gates of Angband after the Dagor-nuin-Giliath (a battle that the Noldor fought against the Orcs just after arriving in Beleriand). Despite the extraordinary abilities of the King of the Noldor, he could not avoid being mortally wounded.
The eldest son of Fëanor, Maedhros, also suffered the terrible power of the Balrogs when he and his delegation were deceived by Melkor and his false proposal of meeting for dialogue. All the Noldor that attended died except for Maedhros himself.
The years passed, and the Balrogs were always in the front row, causing havoc among the Elves despite being locked up in Angband. But when Melkor managed to break the siege to which the Elves and Men had subjected him at the beginning of the Dagor Bragollach, it was the Balrogs and the dragon Glaurung who devastated all the north of Beleriand, destroying Elven fortresses and killing countless Men.
The Balrogs also had a primordial role in the Nírnaeth Arnoediad, the infamous battle in which the Eastern Men betrayed Elves and Édain. Led by Gothmog, the Balrog extended Melkor’s dominions throughout Beleriand, leaving very few places free of his dominion.
In the Fall of Gondolin, the first casualties among the Balrogs took place. Their chief, Gothmog, was killed by Ecthelion, one of the lords of the elven city. Another Balrog was also killed by the elf Glorfindel when the few elven survivors tried to escape the massacre. Both died in the duel.
The remaining Balrogs participated, years later, in the War of Wrath, which was supposed to cause the fall of all the armies of Melkor and the total defeat of the Dark Lord. It is unknown how many Balrogs survived, but some escaped and hid in the depths of the mountains of Middle-earth.
It was not until the Third Age that they awoke. Specifically, one was hidden in the Misty Mountains and was awakened by the dwarves of Khazad-dûm, whose kingdom was ruined and its king Durin VI killed. The Balrog was from then on called Durin’s Bane.
It was Durin’s Bane himself that the Fellowship of the Ring had to face in the part of their journey that forced them to cross Moria (Khazad-dûm) during the War of the Ring. The wizard Gandalf fought against the beast and allowed the Fellowship to escape, but died (although Ilúvatar would resurrect him) along with the Balrog in a fight that took them from the bowels of Moria to the top of the Zirakzigil.
The death of Durin’s Bane ended the history of the Balrog in Middle-earth, whose survivors (if any) were never heard of again.