Balrogs

Balrogs are a powerful demonic race of creatures found in J.R.R Tolkien's fantasy series The Lord of the Rings, they are noted as being a race that have undergone heavy changes by Tolkien as he began to alter them from a race that was once numerous enough to be described as "hosts" to beings that "at most" numbered seven at a time. Another aspect of the Balrog that changed through time was their size and physical nature, with later versions becoming more elemental in nature - fiery and near indestructible (this was the type chosen for the movie, it would appear).

Morgoth
Balrogs were originally Maiar, similar to that of Sauron and Saruman, but after Morgoth's fall into darkness they followed after him and became his demon servants. The creature is reffered to as the Balrog of Morgoth. Morgoth is ultimately the root of all evil in Middle Earth. Gothmog, High-Captain of Angband, was the Lord of Balrogs. During the Fall of Gondolin Gothmog and the Elf Ecthelion fought, both dying when they fell into a Fountain. Another Balrog fought Glorfindel as the Elf protected refugees from Gondolin and they both fell to ruin in the abyss. Most of the remaining Balrogs were slain in the War of Wrath that ended the First Age, though at least one escaped. The demon is encountered in the Mines of Moria, and seems feared by all goblins and orcs that have taken over the tunnels in the mountain.

Durin's Bane lived in the darkness of Khaz-a-dum, known to the Elves as Moria, a deep place of the world, being older and more monstrous than an orc or goblin. In TA 1980 the Dwarves dug too deep for mithril and woke the Balrog. It slew the Dwarf King Durin VI, and the next year Durin's son Nain I. The Dwarves were driven out of Moria. In 2799 at the Battle of Azanulbizar the Dwarf Dain, later Dain II, King of Durin's Folk, when he pursued the Orc Leader Azog to the Gate of Moria and slew him, saw Durin's Bane. He told his father's cousin Thrain II, that Moria could not be taken.

The Lord of the Rings' Encounter
The Lord of the Rings contains only a brief encounter with the demon; this particular Balrog had no true name and was refered to as Durin's Bane. In TA 3019 the Fellowship of the Ring are forced to enter Moria. The Balrog scares off the goblins surrounding the fellowship then follows them to the bridge in Khaz-a-dum. The fellowship of the ring make it across the bridge, but Gandalf stays, and prevents the beast from crossing, breaking the Balrog's sword. He uses his magic to destroy the bridge, but is pulled down by the whip of Durin's Bane. The Balrog is eventually slain by Gandalf on the mountains of Charadras.