The area where the river Narog flowed into Sirion, on the southern edge of Beleriand. After the NirnaethArnoediad, the Orcs and wolves of Morgoth roamed as far south as Nan-tathren. The refugees of Gondolin led by Tuor fled to and rested here where the power of Ulmo was still strong before the Sea-longing awoke again in Tuor.
Nan-tathren means literally 'Willow-vale', but is translated as 'the Land of Willows'. The Quenya forms were Tasarinan and Nan-tasarion, used in Treebeard's song in The Two Towers.