Thargelion was a land of Beleriand.
Thargelion literally meant 'beyond Gelion', as it was east of the river Gelion. It was north of the river Ascar, and therefore not counted as part of Ossiriand.
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.
The Haladin briefly dwelt here until they were nearly wiped out in an Orc-raid.