The Elven Kingdom of Hithlum is located inside of the Ered Wethrin. It is a Noldorin kingdom, ruled by High King Fingolfin of the Noldor. The cavalry of Hithlum is said to be one of the best in all of Middle Earth, and it's archers were unmatched.
|“||Fingolfin and Fingon his son held Hithlum, and the most part of Fingolfin's folk dwelt in Mithrim about the shores of the great lake; to Fingon was assigned Dor- lómin, that lay to the west of the Mountains of Mithrim. But their chief fortress was at Eithel Sirion in the east of Ered Wethrin, whence they kept watch upon Ard-galen; and their cavalry rode upon that plain even to the shadow of Thangorodrim, for from few their horses had increased swiftly, and the grass of Ard-galen was rich and green.||”|
–Quenta Silmarillion, Of Beleriand and Its Realms
Hithlum was first populated by the Elves of Mithrim, who were Sindar. The Ñoldor entered Hithlum via the Firth of Drengist and first camped at the shores of Lake Mithrim. The land was covered by mists and vapours from the Thangorodrim, and thus they named it Hísilómë or Land of Mist. Thingol gave leave to the Noldor to settle it and was ruled by Fingolfin and Fingon from its eastern lands.
Later in the First Age, Hithlum was continually under attack by Morgoth, but its natural defenses, fortified by fortresses such as Barad Eithel, proved impassable. Almost a century after the Dagor Aglareb, a small number of Orcs came from the Firth of Drengist from the west, but once they were spotted, Fingon pushed them back into the sea.
During the following peaceful years, Fingolfin welcomed to his service the folk of the House of Hador to whom he gave some lands. The mountains protected Hithlum during the Dagor Bragollach, after which Edain of the First House came from Dorthonion as refugees and joined those of Hador.
Seven years after the Dagor Bragollach, Morgoth sent a great force against Hithlum, attacking the passes of the Ered Wethrin. Galdor died defending the Eithel Sirion; in a second attempt the Falathrim sailed up the Firth of Drengist, then assisted Fingon by striking the unsuspecting orcs from the west.
Hithlum eventually was lost after the Nirnaeth Arnoediad. The Hadorians were scattered, killed, or enslaved, the Noldor who could not flee in time were enslaved in Morgoth's mines. Morgoth gave the Swarthy Men the lands of Hithlum as a reward, in actuality they were trapped there.
Hithlum was completely destroyed during the War of Wrath.