The greatest and most powerful of all the Balrogs. He was the High-Captain of Angband and the Lord of Balrogs. As a servant of Morgoth, he was second only to Sauron. Most of the time he was accompanied by other balrogs, and in the Battle of Unnumbered Tears he had a guard of trolls. Unlike most balrogs, his weapon was a giant black axe.

He was first encountered by the elves in the Battle Under the Stars (Dagor-nuin-Giliath) in year 1 of the First Age. Here Fëanor had fought so mightily that he was separated from the host of Elves, and had just a small band with him. They were surrounded by balrogs, and Gothmog mortally wounded him.

The next time Gothmog was mentioned was in the Battle of Unnumbered Tears (Nirnaeth Arnoediad) in year 471 of the First Age. Then he came from Angband with a large army, and surrounded Fingon. Fingon fought bravely, but was killed when another balrog trapped him with his fiery whip, letting Gothmog strike with his axe. Gothmog's army of trolls then captured Húrin, and Gothmog dragged him back to Angband.

Gothmog was also part of Morgoth's assault on Gondolin. In the King's Square in the middle of the city he met Ecthelion of the Fountain. There they fought, and both died in the duel.