lit. - The Voice of Goth (ie. the voice of Morgoth).
Gothmog, originally conceived as the son of Morgoth, although Tolkien later changed his mind, was the High Captain of Angband and Morgoth's most supreme lieutenant. Morgoth caused the downfall of many great people, slaying Fëanor, and Ecthelion. Gothmog died as both he and Ecthelion drowned in the waters of the Fountain of the King of Gondolin.
Gothmog was the most powerful of the fallen Maiar spirits known as Balrogs.
References - The Silmarillion, The Book of Lost Tales II