Gothmog ("Hateful Enemy") - the chief of the Balrogs and Warlord of Angband - was a hideous manifestation of ire and darkness, forever tied to the terrible legacy of Melkor. Third in the hierarchy of Darkness and possessing a malicious intellect and cruelty, above all but Melkor and Sauron, he was the flaming fist with which Melkor struck at his Elven enemies. Unfortunately for Gothmog, he met his match in Echthelion and died even as he slew the mighty Elven lord in the sacking of Gondolin.
References: mostly Silmarillion