One of the great power, a veritable demon of fire
who under the stars slew elven sire
when the tears had no numbers he slayed the king
and was himself slain by elven sting.
Gothmog
Gothmog was the Lord of Balrogs and the High-captain of Angband, one of the chief servants of the Dark Lord Morgoth with a rank equal to that of Sauron.
Killed Feanor in Dagor-nuin-Giliath (Battle Under Stars), Killed Fingon in Nirnaeth Arnoediad (Battle of Unnumbered Tears), and was killed by Ecthelion in Gondolin.
This is a different Gothmog than the Lieutenant of Morgul in the Lords of the Rings, who was named after Gothmog the Balrog.
Tolkien Gateway: Gothmog (Balrog)