Dorthonion, later Taur-nu-Fuin (also called Orod-na-Thon and Deldúwath), was a highland region of the First Age lying immediately to the north of Beleriand, and south of the plains of Ard-galen (later Anfauglith) bordering Morgoth's stronghold of Thangorodrim.
Dorthonion was 60 leagues east to west. In the north, it rose gradually from the plains, with great pine forests. The Ered Gorgoroth formed the southern boundary of Dorthonion, bending to the north on the east side to create the Pass of Aglon between Dorthonion and Hills of Himring. To the south and west of Dorthonion were the Echoriath, which surrounded the hidden Kingdom of Gondolin. The upper reaches of the River Sirion passed between the western slopes of Dorthonion and the Ered Wethrin.
Dorthonion was originally ruled by Angrod and Aegnor, sons of Finarfin, but it was overrun by Morgoth in the Dagor Bragollach, after which it was known as Taur-nu-Fuin which translates as Forest under Night or The Forest under the Shadow of the Night.
The northeastern slopes of Dorthonion were known as Ladros, a fief given to Boromir of the House of Bëor. Here Barahir and a small band of outlaws dwelt until they were all slain at Tarn Aeluin by Orcs sent by Sauron, save for Beren, Barahir's son in FA 460. Beren dwelt alone in the wilderness of Dorthonion for a time until he left for Doriath and met Lúthien.
Along with the other lands west of the Blue Mountains, Taur-nu-Fuin was mostly destroyed in the War of Wrath at the end of the First Age. Afterwards, its highest points survived as the island Tol Fuin (Isle of Night), part of the Western Isles.