Balrog in the New Line film
Evil beasts opposing any good force. These devil-like creatures were made of fire and rock. They lived in the deepest, darkest places inside mountains. They were originally Maiar, servants of the Valar. The species was influenced by Morgoth to turn to his side. They lived in the depths of Angband, listening for Morgoth's call for aid. His first call was when Ungoliant the evil spider came to steal Morgoth's Silmarils from his Iron Crown. The Balrogs defeated Ungoliant and went back into hiding. Later, in 1497 of the Years of the Trees, Feanor came to Angband to get his Silmarils. Balrogs fought him off and captured his eldest son Maedhros. Gothmog the Balrog was soon named their Lord. He led armies of Balrogs to the Battle of Sudden Flame and the Battle of Unnumbered Tears. But in the War of Wrath, Morgoth was defeated and all but one Balrog were slain. This Balrog went to the mountain cave of Moria and hid. It was in 1980 of the Third Age that the peoples of Middle-Earth became aware of its existence. The Dwarves of Khazad-Dum awoke it and it killed many including their King Durin VI. This caused the Dwarves to evacuate the next year. 1,009 years later, Balin led a rather large company to Moria to mine. They were all killed after five years. On January 13, 3019, The Fellowship of the Ring entered Moria and accidentally made a great deal of noise. This startled the Balrog and caused him to follow them to a section of the endless stair. The Balrog captured Gandalf and they fought as they fell from the Bridge of Khazad-Dum. The Balrog was eventually defeated in the Battle of the Peak. It can be supposed that Balrogs went extinct, though some may have lurked elsewhere in Middle-Earth.