Wikipedia
In the fictional world of J. R. R. Tolkien's Middle-earth, Dorthonion ("Land of Pines"), later Taur-nu-Fuin, 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) that extended north to Morgoth's stronghold of Thangorodrim. It is notable in its function as a dangerous stage and scene in the adventures of many major characters in several of Tolkien's books and other works such as The Silmarillion, The Lays of Beleriand, and most recently The Children of Húrin. Similar to the other great forests of Tolkien's legendarium such as Mirkwood, Fangorn and Lothlórien it provides a transitional device in the invented history of Tolkien's Middle-earth and important episodes in the heroic quests of his characters such as Beren, Lúthien, Beleg and Túrin.
Dorthonion was 60 leagues east to west. In the north it rose gradually from the plains, with extensive pine forests on these slopes, as well as on the western slopes above the Pass of Sirion. The majority of Dorthonion consisted of a high plateau with bare and rocky peaks rising to higher altitudes than the mountains of the nearby Ered Wethrin (the Shadowy Mountains). The Ered Gorgoroth (Mountains of Terror) 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 (Encircling Mountains), which surrounded the hidden kingdom of Gondolin. Between Dorthonion proper and the Echoriath lay the Pass of Anach.
Dorthonion was originally ruled by Angrod and Aegnor, two of the sons of Finarfin, under the suzerainty of their older brother Finrod. The north-eastern area of Dorthonion, Ladros, was given to Boromir of the House of Bëor as a fief (c. Y.S. 350) and held by his descendants.
In the Dagor Bragollach, Dorthonion was a key theatre of war, as Morgoth concentrated on conquering it. Angrod and Aegnor were killed, along with most of their people, including Bregolas, then Lord of Ladros, and virtually all the warriors of his House, and Dorthonion was overrun. Bregolas' brother Barahir remained in Dorthonion leading a band of guerrillas in opposition to Morgoth, and retreated to the high mountains of Ladros. The pine forests, under Morgoth's influence, gradually became dark and dangerous, and were named Taur-nu-Fuin (the Forest under Nightshade); it was said that the forest was haunted, and that those who entered would be trapped and lost, or driven mad with terror. As Barahir's forces were driven back, and more and more of Dorthonion fell to Morgoth (a process which was complete after a few years), the name Taur-nu-Fuin was increasingly applied to the whole plateau.
Allied forces under Maedhros regained Dorthonion shortly before the Nírnaeth Arnoediad, before Morgoth took it back permanently in the aftermath of that battle.
Along with the other lands west of the Ered Luin, Taur-nu-Fuin was mostly destroyed in the War of Wrath at the end of the First Age. Its highest parts survived as the island Tol Fuin (Isle of Night), part of the western isles.