Balrogs

Balrogs alson known as the Valaraukar were Maia spirits corrupted by the will of Melkor and forced to serve him.

Ancient times
They were Maia spirits who were persuaded by combined might and splendor of Melkor and Mairon to join their cause. They first dwelled in the fortress of Utumno, but after the Dark Lord's defeat during the War for Sake of the Elves, the Balrogs and other dark creatures escaped and went to Angband. Until their master's return they seem to have obeyed Sauron, who was the greatest of Melkor's servants at that time.

Years of the Trees
Balrogs were present as early as the Years of the Trees when Melkor and Ungoliant went to Valinor and destroyed the Two Trees. . After the two destroyed the Trees, they came to the ruins of Angband to renew the fallen Vala's rule in Middle-earth. A disagreement with Ungoliant led to her attacking Melkor, and hegave out a great cry that roused the Balrogs from their slumber. In a tempest of fire, the demons drove Ungoliant away and prepared to pursue her. However, they were halted by Melkor and returned to Angband, which shortly thereafter was constructed anew.

First Age
When the Noldor won the battle Dagor-nuin-Giliath,  Fëanor furiously pressed on toward Angband. He came even within sight of Angband, but was ambushed by a force of Balrogs with few Noldor around him. Soon he stood alone, but long he fought on with all Balrogs alone as mightiest all the Children of Iluvatar even though he was wrapped in fire and wounded with many wounds. But finally Gothmog the Lord of the Balrogs, felled and mortally wounded Fëanor.

The Balrogs fought during the Nírnaeth Arnoediad, where Gothmog led the invasion. He threw aside Húrin and Turgon, turned upon Fingon and killed him with the help of another Balrog, securing the field for Melkor's forces. He also captured Húrin, after Húrin was buried under a mountain of slain foes. He bound the human warrior and delivered him to Angband, whereupon the Dark Lord attempted unsuccessfully to pry the location of Gondolin from him.

In FA 510, during the Fall of Gondolin, the Balrogs rode upon the backs of dragons to reach the hidden city of Gondolin. The Lord of the House of the Fountain, Ecthelion, managed to kill Gothmog at the cost of his own life. While attempting to escape the burning city, Glorfindel and his companions were blocked by another Balrog. To save Tuor, Idril and their young son Eärendil, Glorfindel fought the Balrog on a cliff and cast it down, but he was pulled down with the Balrog to their deaths.

The surviving Balrogs fought in the War of Wrath which saw the defeat of the Dark Lord Melkor and the rise of Sauron to power. They hid in Earth's deep caverns waiting to be called back by a returned Morgoth.

Third Age
In TA 1980, a Balrog awoke in Moria when the Dwarves had mined too deep for Mithril. It drove the Dwarves out of their home and slew King Durin VI, and the Balrog was thereafter called "Durin's Bane" After this, Sauron began to "people" Moria with Orcs and Trolls so as to control it, through it is unknown if the creature was even aware of the Dark Lord and thus the extent of their alliance remains unclear.

During the War of the Ring, the Fellowship of the Ring passed through Moria and encountered Durin's Bane, which pursued them to the Bridge of Khazad-dûm. Gandalf the Grey fought the Balrog, allowing the Fellowship to escape Moria.[11]Both fell into the abyss, but the battle continued at the peak of Zirakzigil. Finally, it ended, but both Gandalf and Durin's Bane were slain in the process.

Appearances

 * The Silmarilion
 * The Lord of the Rings: The Fellowship of the Ring
 * The Lord of the Rings: The Fellowship of the Ring (film)
 * Unfinished Tales