Tol Sirion later known as Tol-in-Gaurhoth (Isle of Werewolves) was an island in the upper reaches of the river Sirion between the Ered Wethrin (Mountains of Shadow) to the west, the Echoriath (Encircling Mountains) to the east, and the Fen of Serech to the north.
The exiled Ñoldor of the House of Finarfin under Finrod first showed interest in the isle between the years 50 and 60 of First Age, when Finrod raised Minas Tirith to prevent the armies of Morgoth from passing through the area between the mountains and invading the Beleriand. Finrod committed the isle to Orodreth, his brother who ruled it until Sauron conquered it after the Dagor Bragollach and turned it into a haven for his Werewolves, calling it Tol-in-Gaurhoth.
Later on the Quest for the Silmaril, Beren and Finrod were captured by Sauron's forces passing the isle. After Finrod and his companions were killed, Lúthien and Huan were able to defeat Sauron's forces there and they fled. The isle was cleansed and remained so but was no longer inhabited.