Gothmog, the Lord of the Balrogs
Gothmog, Lord of the Balrogs, is one of the Maiar who, at the beginning of time, chose to serve Melkor. By virtue of his strength and violence (and his total lack of honor and loyalty), he assumes command of the armies of Angband, becoming Morgoth's greatest lieutenant alongside Sauron.
He therefore leads the armies of Angband in several battles in Beleriand against the Elves, and it is he who kills Fëanor during the Dagor-nuin-Giliath and Fingon during the Nirnaeth Arnoediad. And it is he who leads the assault on the city of Gondolin, where he meets his death at the hands of Ecthelion of the Source. Gothmog is therefore a memorable character and a terrifying adversary, whom Tolkien had initially intended to be a direct descendant of Melkor.