The “Mountains of Shadow” west of Mordor (LotR/636), or more literally the “Fence of Shadow” (RC/457). This name is combination of ephel “fence” and dúath “night shadow” (SA/pel, dú, gwath; PE17/152).
Conceptual Development: In Lord of the Rings drafts from the 1940s, the earliest Elvish name these mountains was simply N. Duath (TI/309, TI/343), later revised to N. Hebel Dúath (WR/122) and finally Ephel Dúath (WR/180).
ephel (“outer fence, encircling fence”) < ed (“outer”) + pel (“fenced field”), dû (“nightfall, dimness”) + gwath (“shade”)