Thargelion, earlier known as Talath Rhúnen and later known as Dor Caranthir, was a flat wooded region of East Beleriand, between the river Gelion and the Blue Mountains. It was north of the river Ascar, and therefore not counted as part of Ossiriand. At its northern end was Lake Helevorn and Mount Rerir.
The Sindar called this land Talath Rhúnen. After the Rebellion of the Noldor this was the land of Caranthir, one of the Sons of Fëanor, and after him it was often called Dor Caranthir, "Land of Caranthir". There the Noldor met the Dwarves of the Blue Mountains.
The Haladin briefly dwelt there independently, near the river Ascar, until they were nearly destroyed in an Orc-raid. Caranthir offered part of his land to their leader, Haleth, but she refused and instead led her people to Brethil.
During the Dagor Bragollach, Orcs broke the defences of Maglor's Gap, defiled Helevorn, and ravaged all Thargelion before passing over Gelion farther into East Beleriand. Caranthir fled to the south with the remainder of his people.
Caranthir later gave some of his former lands to Uldor.