Balrogs were originally Maiar, commonly known as wizards (i.e. Gandalf, Saruman, Radagast), but decided to side with Morgoth, the dark and powerful Ainur (gods). It is believed their corruption led to their demonic appearance over the thousands of years.
How many balrogs existed is debatable. In some of Tolkien's text, he describes an army of balrogs led by Gothmog. In later writings, it is said there were only ever seven total. Regardless, all of them were thought to be dead after the War of Wrath at the end of the First Age, but a surviving balrog was discovered by the dwarves of Moria, which led to their demise. This balrog, commonly known as Durin's Bane, is easily the most well known balrog in all of Tolkien lore. He was eventually slain by Gandalf the Grey, but Gandalf himself died in the process.
Appearances in Games
Balrogs have appeared in many of The Lord of the Rings video game titles. Games such as The Battle for Middle-earth, The War of the Ring, and The Lord of the Rings: Conquest all contain one of the fearsome beast. In the real-time strategy games based on The Lord of the Rings universe, the balrogs are one of the toughest units on the battlefield. If not stopped quickly a balrog can deal out a massive amount of damage on units and structures under the player's control. Players have had the chance to take control of this creature in a more action oriented fashion in The Lord of the Rings Conquest. In this game, the balrog is one of the largest enemies that players may encounter. When playing as the balrog, players can easily take out enemies and leave the opposition in disarray if played properly.