The name of the Balrog that Gandalf fights in The Lord of the Rings: The Fellowship of the Ring is Durin’s Bane. Gandalf defeated his enemy on top of Zirakzigil—the highest peak in the Misty Mountains. Although the fiery demon is the only one seen in the movie, there are actually many Balrogs. This particular beast was awoken by the Dwarves who dug too deep in the mines within Moria.
These creatures are very large, recorded as being twice the height of an Elf with a whip and sword made of fire that never stopped burning and even appeared lava-like. It is not known for sure, but some believe the creatures have wings. And despite looking like a proper demon in The Fellowship of the Ring with horns and all, the Balrogs are actually described as being humanoid in shape. But the movie does show how Balrogs are made of fire and shadow, and like their weapons, they too are always burning.
Tolkien—creator of The Lord of the Rings—was a writer and a storyteller. And like all great writers invested in their work, the story was ever-changing. At one point, Tolkien said there were thousands of Balrogs, and although they were tough and it wasn’t easy to kill them, they were much more destructible than Durin’s Bane. But in one of Tolkien’s books, the author wrote in the margins a note that was shared by his son, Christopher Tolkien. The note read, “There should not be supposed more than say 3 or at most 7 ever existed.”
Balrogs have another name they are known by to those in Middle Earth, and that is Valaraukar. These creatures were Maiar, and for anyone not familiar, Maiar (or Maia in singular form) can be seen as the angels of Tolkien’s writings. Angels in Christianity are servants of God and perform tasks like monitoring each and every person, including all their actions and giving a report to God. They also relay messages from God. The Valar are those who first shaped and created the world, and the Maiar were created to help them do this. Balrogs used to be these angel servants to the Valar, but they were corrupted by Melkor—the first Dark Lord.
When Melkor was defeated a first time, the Balrogs escaped and went to hide in Angband, which was the Dark Lord’s fortress. In the shadowy pits, they slept until awoken by Melkor, who let out a loud cry after being attacked by the spider Ungoliant (Shelob’s mother). During the War of Wrath, where Melkor would be defeated a final time, the Balrogs participated in the battle and were mostly destroyed. A few survived, but they crept back into the shadow of Earth’s deep caverns to hide. After this, during the Second Age, Sauron began to take up his master’s work and finish what Melkor started. Although he was gathering all the evil left in the world, it is not known for certain if the Balrogs were included.
The Balrogs were dormant for thousands of years underground beneath the Misty Mountains until one of them—Durin’s Bane—was awoken by the Dwarves. It is not known how many other Balrogs were present or active, but this Balrog had the most significance to the story as he was known to be active in the Mines of Moria after killing King Durin VI and being spotted by Thráin II, both of the Dwarven race. It’s not like there were telephones and mailmen in The Lord of the Rings, but word of mouth did travel, so this is probably how Gandalf knew that the Dwarves had awoken Durin’s Bane. Gandalf is also a Maia, so maybe he could sense other Maiar spirits as well.
In the movie, Gandalf slays his enemy after falling into an underground river where the Balrog’s flames were put out, and it became a slimy, shadowy monster that kept grabbing at Gandalf. How did Gandalf and the Balrog get to the top of the mountain peak after falling in this river? The Balrog needed the sunlight to “recharge” its flames, so it ran away from Gandalf into the tunnels of the dungeon. Gandalf followed his enemy because he would be lost in the maze without it. The two Maiar reached the Endless Stair, which was a spiral staircase built by the Dwarves that ran the distance of the lowest dungeon all the way to the highest peak, unbroken.
In the process of slaying his enemy, Gandalf was also slain—or possibly killed by using up all his energy and spells to fight off the beast. But, when his spirit returned to the Undying Lands, the Valar sent him back to complete his task as Gandalf the White. Gandalf was scared of this beast because he knew how powerful it was since it was a Maia like himself, but it was purely evil.