Ephel Dúath

The Elvish names of mountain ranges typically contain the word ered, the plural of orod, 'mountain' (as in Ered Gorgoroth, Ered Lithui, Ered Luin, and many other examples). The ephel in Ephel Dúath literally means 'outer fence' (as opposed to the inner 'fence' of the Morgai, presumably). This use of the word for a mountain range is unique, but it does also appear with a more literal meaning in Ephel Brandir, the name of the stockade on Amon Obel in Brethil.

To confuse matters, the Ephel Dúath weren't the only Mountains of Shadow in Middle-earth. The name was also applied to the far northern range known in Elvish as the Ered Wethrin, though those northern mountains had been lost beneath the Sea at the end of the First Age.