Mirkwood (from the Grey-Elven Taur e-Ndaedelos, ‘Forest of Great Fear’) was the name given to the mightiest surviving forest of western Middle-earth, formerly known as Greenwood the Great. The forest had been renamed after the raising of the evil tower of Dol Guldur. With the growing power of Sauron at Dol Guldur, the forest became inhabited with Orcs, black squirrels, and great spiders. When Thorin Oakenshield and Company passed through Mirkwood in TA 2941, they encountered the great spiders and other strange insects, an enchanted stream, and a feeling of darkness and oppression.
Following the War of the Ring, Mirkwood was cleansed and renamed Eryn Lasgalen (S. ‘Wood of Green Leaves’).