Balrogs

Balrogs are creatures that were gods but were corrupted by Morgoth, a different god and turned into ugly, powerful demons of shadow and flame that have whips of fire. They are huge and have burning yellow eyes. No one is certain on whether or not the balrogs had wings. They were originally spirits of fire. When Morgoth stole the silmarils he got trapped in the web of the Great Spider, Ungoliant but cried out and the balrogs came and freed him with their fiery whips. Then the balrogs killed Feanor, the creater of the silmarils. During the Battle of Unnumbered Gothmog, Lord of the balrogs, killed Fingon, High King of the Noldor, as the rest of the balrogs beat him with maces. Gothmog battled the Elf-lord Ecthelion and they both were killed. Another balrog battled Glorfindel and both of them fell into the abyss to their deaths. When the Valar vanquished Morgoth in the War of Wrath, most of the balrogs were killed but a few of them escaped to the Misty Mountains. One of these was the balrog that chased the elves out of Khazad-Dum. After that orcs and other creatures went to dwell there. The same balrog would later destroy a dwarf colony and would engage in a battle against Gandalf while trying to kill the Fellowship of the Ring. Gandalf shattered the bridge underneath him and he fell but caught Gandalf and dragged him down with him. He burned Gandalf on the way down. And when they finally were down to the ground again, Gandalf threw the balrog into the mountainside and killed him. Gandalf would die too but would later return to life as Gandalf the White. Since then there hasn't been any balrogs.

Source: "Other Creatures of Middle Earth" http://www.tuckborough.net/creatures.html