The Balrogs were spirits of fire seduced by Melkor in the beginning of Arda, and who dwelt in his northern fortress of Utumno.
During the First Age, the Balrogs were the most feared of Morgoth's forces after the dragons.
In appearance, the Balrogs were man-like, but fire streamed from them, and they were swathed in dark shadows. They carried whips of flame and induced great terror in friends and foes alike.
In the War of Wrath, Morgoth was assailed by the forces of the Valar. Most of the Balrogs were destroyed in that War, but some few escaped over the Blue Mountains and lay hid in Middle-earth. Durin's Bane, the creature that drove the Dwarves from Moria, was one of these.
The Balrogs Before the First Age
The Balrogs were in origin Maiar, of the same order as Sauron or Gandalf. Melkor corrupted them to his service in the distant past of the World, in the days of his splendour. They were originally gathered by him in his ancient fastness of Utumno during the time of the Lamps of the Valar. When this fortress was destroyed by the Valar, at least some fled and lurked in the pits of Angband (whether any of the original Balrogs were slain in the Valar's attack on Utumno is not known).
Balrogs in the First Age
When Melkor and Ungoliant escaped from Valinor three ages later with the Silmarils, the Balrogs were still to be found in the ruins of Angband. Ungoliant entrapped Melkor in her webs, demanding the Silmarils for herself, but the Balrogs issued from their hiding-place and rescued their lord.
The Balrogs were apparently first encountered by the Elves during the Dagor-nuin-Giliath in the first year of the First Age. After the great victory of the Noldor over Morgoth's Orcs, FŽanor pressed on towards Angband, but the Balrogs came against him. He was mortally wounded by Gothmog, Lord of Balrog. Though his Sons beat off the demons of fire, FŽanor died of his wounds soon after, and his spirit departed for the Halls of Mandos.
The Appearance and Nature of Balrogs
Balrogs were spirits of fire - their hearts were of fire, we are told, and they carried whips of flame. They could, however, shroud themselves in darkness and shadow. The Balrog that Gandalf fought in Moria, for example, at first gave no hint of his fiery nature apart from the flames that issued from his nostrils.
(Lord of Balrogs) entered Arda at its beginning, slain I 510
Titles: High-captain of Angband, Lord of Balrogs
The most powerful of all the Balrogs, one of the chief servants of Melkor, who held an authority hardly less than Sauron himself. A wily commander and fearsome fighter, Gothmog was often accompanied by others of his fiery kind, and at least in the Nirnaeth he had a personal guard of dozens of trolls. His weapon was a great black axe.
The Elves first encountered him in the Dagon-nuin-Giliath, the battle fought under the stars before the first rising of the Moon. In that battle, FŽanor's fury had caused him to draw forward of the main force of the Noldor, and so he came upon Gothmog with only a small guard. Even so, he fought valiantly, and though the armies of the Noldor eventually drove the Balrgos off, FŽanor's wounds could not be healed.
After that first battle, Gothmog does not appear for several centuries. He emerged from Angband to fight in the battle known as the Nirnaeth Arnoediad, the fifth and last battle of the Wars of Beleriand. He brought up a huge army, and drove aside the forces around High King Fingond, who he slew with his axe. His trolls assailed Fingon's ally Hķrin, who he eventually captured and brought back to Angband.
But Gothmog was not indestructible. He took part in (and indeed presumably led) Morgoth's deadly surprise assault on the Hidden
City of Gondolin. In the square of the King in the heart of the city, he came upon the Elf-lord Ecthelion of the Fountain. They
fought a great duel, and in the end Gothmog and Ecthelion slew one another. So ended one of the most feared denizens of the pits