Belegost (S. ‘Mighty fortress’) was the Elves’ name for the Dwarf-city of Gabilgathol, built early in the First Age and located east of the Blue Mountains. To Men, it was known as Mickleburg. From here and the other great city of Nogrod, the Dwarves ventured into Beleriand to trade and explore; thus, they were the first new speaking-peoples the Sindar encountered. King Thingol soon sought their counsel, and the Dwarves aided him in the building of Menegroth (the ‘Thousand Caves’), for which they were given the great pearl Nimphelos.
Azaghâl was the first recorded name of any King of Belegost; he brought his people to the Nirnaeth Arnoediad, where they fought in alliance with the Sons of Fëanor. While Azaghâl was killed in the fight, it was his people who defeated the dragon Glaurung.
The city of Belegost was ruined in the Great Battle at the end of the First Age; many of the Dwarves went to Khazad-dûm. In addition to Menegroth being one of their greatest works, the Dwarves of Belegost also invented chainmail.