Balrogs, or “Demons of Might” (loose translation in Elvish), are primordial spirits. Originally Maiar, these fiery demons carried swords and whips, moving quickly. Balrogs did have some mastery over magical arts. Most of the Balrogs were destroyed in the War of Wrath, a few of them retreating into the Blue Mountains and hiding in Middle-Earth.
Balrogs were first encountered by the Elves in Dagor-nuin-Giliath. Regarding their origins, they were corrupted by Melkor, who gathered them in Utumno during the Lamps of the Valar. Following the Fall of Gondolin, Balrogs were rarely mentioned until the War of Wrath. Towards the end of the First Age, the Valar loosened the power of the Balrogs in Angband, causing them to flee to Misty Mountain.
During King Durin IV’s reign, the Balrogs were dormant for nearly five thousand years in the Mountains of Moria. Sauron’s raising of Dol Guldur in the Third Age saw the reawakening of the Balrogs. In the Third Age, 1980, a Balrog was discovered by the Dwarves of Khazad-dum in their search for mithril. Inevitably, Khazad-dum would be laid to ruin and Durin slain. Following these events, the Balrogs remained quiet in Moria for hundreds of years.
In the Fellowship of the Ring, a Balrog challenged Gandalf the Grey, a fellow Maia. Their battle took them to the depths of Moria, up the Endless Stair, where Gandalf finally slayed the Balrog atop the peak of Silvertine. Of the known Balrogs, Gothmog was considered the Lord of the Balrogs.