Slayer of the joint Eleven King.
In dark bardoc poems his deeds they did sing.
Slayer again of the noblest elf,
til slain by elf himself.
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.
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)