Gothmog (Sindarin IPA: [ˈɡoθmoɡ]) was the Lord of the Balrogs during the First Age and was the greatest Balrog ever to walk Middle-earth.
He is the tertiary antagonist in The Silmarillion.
He was the High Captain of Angband, one of Morgoth's most trusted generals, leading the Dark Lord's forces to victory in countless Battles. His only equal in rank was Sauron. Gothmog, like the rest of his kind, carried a fiery whip into battle, but he was also known to have wielded a great black axe, which was feared by elves across the continent of Beleriand.
Breaking the Siege of Angband
Gothmog fought in numerous battles in the War of the Jewels and led the charge in the Battle of the Sudden Flame when he broke the Siege of Angband. Gothmog may have been the one who caused flame to spew over the Thangorodrim volcanoes, because of his pyrokinetic nature.
Gothmog also had a part in the Battle of Unnumbered Tears, where he slew countless elves and humans. Gothmog had an alliance with three treacherous Easterling men, who betrayed their allies by stabbing them in the back at the height of battle.
Conquering Gondolin and Death
During the time of Earendil, Gothmog led the Siege of Gondolin, taking it unawares during a festival. He razed large portions to the ground and overran it with his Orcs, dragons, demons and monsters. They were assisted by the King's treacherous nephew Maeglin, who gave them the location of the city. Gothmog led all his Balrogs into the battle. He was successful in completely destroying Gondolin, and was duelling Ecthalion, the leader of the House of the Fountain, who fought the demon lord in close combat in the Fountain Square. Finally Ecthelion plunged the diamond-tipped spike on his helmet into Gothmog, and they fell together into the fountain. Gothmog's fire was quenched and both died.
Gothmog's Quenya name was Osombauko (or Oþombauko; Tengwar: full spelling `N3Hw.EaY or vowel-abbreviated spelling `N3Hw.aY; IPA: [osomˈbaʊko] or Vanyarin [oθomˈbaʊko]).