A range of peaks that rose out of the dense woodland known in earlier times as Greenwood the Great, and later as Mirkwood. The range ran for some fifty miles from end to end, and its slopes were covered in fir-trees that cast the ground below into deep shadow. The shadows of the trees gave rise to the former name for these highlands: Emyn Duir, or the Dark Mountains.

The Silvan Elves of the Greenwood had not at first dwelt on the heights, but as a Shadow began to spread northward through the trees, the Elves removed northward. The settled for a time on the western slopes of the Dark Mountains, and in the woods to the westward, but these Wood-elves did not remain in the mountains for long. As the Shadow continued to spread, they abandoned the heights and moved farther northward still, founding the kingdom of the Woodland Realm in the far northeast of the Wood.

With the coming of the Shadow, the darkened forest had become known as Mirkwood, and the Dark Mountains in turn became Emyn-nu-Fuin, the Mountains of Mirkwood. Where the Elves had once dwelt on their slopes, the creatures of Necromancer now roamed. The river that rose in the heights turned black, and its cursed waters ran down northward from the mountains far beneath the trees.

The Forest was shrouded in the Shadow from Dol Guldur through some two thousand years until the time of the War of the Ring. Sauron was finally defeated in that War, and his fortress of Dol Guldur was razed. Afterward, the cleansed forest of Mirkwood was renamed as Eryn Lasgalen, the Wood of Greenleaves, and the Mountains of Mirkwood must surely also have been renamed. Whether they reverted to their old name of the Dark Mountains, or (like the Forest itself) received an entirely new name, we are not told.