The Mountains of Mirkwood
(This item draws substantially on the Tolkien Gateway article of the same name.)
In the central parts of northern Mirkwood, north of the Old Forest Road, a jumble of fir-covered, low-lying hills to the west rose to reach greater heights in the east, together forming a range nearly one hundred miles in length. Historically, the Second Age saw these mountains inhabited by Silvan Elves, who knew the range as the “Emyn Duir”, the “Dark Hills” (or Dark Mountains) because of the dense fir-woods that grew upon their slopes — but with the attribution of ‘dark’ not originally implying ‘dark’ in the sense of evil.
During the Third Age, when the Necromancer (Sauron) caused The Shadow to spread across the mountains as it did throughout the rest of the Forest, they became populated by hideous creatures. Just as Greenwood the Great was renamed Mirkwood at this time, its northern hills came to be called “Emyn-nu-Fuin”, literally the “Hills under Shadow” but usually rendered as the “Mountains of Mirkwood”.1 They seem to have become more of a focus for evil than the rest of the forest about their feet, with fell beasts and darker things issuing out the Mountains to pass down into Mirkwood.
1 J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, “The Disaster of the Gladden Fields”, Note 14