Gothmog

The most powerful of all the Balrogs, one of the chief servants of Melkor, who held an authority hardly less than Sauron himself. A wily commander and fearsome fighter, Gothmog was often accompanied by others of his fiery kind, and at least in the Nirnaeth he had a personal guard of dozens of Trolls. His weapon was a great black axe.

The Elves first encountered him in the Dagor-nuin-Giliath, the battle fought under the stars before the first rising of the Moon. In that battle, Fëanor's fury had caused him to draw forward of the main force of the Noldor, and so he came upon Gothmog with only a small guard. Even so, he fought valiantly, and though the armies of the Noldor eventually drove the Balrogs off, Fëanor's wounds could not be healed.

After that first battle, Gothmog did not reappear for several centuries. He emerged from Angband to fight in the battle known as the Nirnaeth Arnoediad, the fifth and last battle of the Wars of Beleriand. He brought up a huge army, and drove aside the forces around High King Fingon, and slew the King with his axe. His Trolls assailed Fingon's ally Húrin, who he eventually captured and brought back to Angband.

But Gothmog was not indestructible. He took part in (and indeed presumably led) Morgoth's deadly surprise assault on the Hidden City of Gondolin. In the square of the King in the heart of the city, he came upon the Elf-lord Ecthelion of the Fountain. They fought a great duel, and in the end Gothmog and Ecthelion slew one another. So ended one of the most feared denizens of the pits of Angband.