im (“between, within”) + lad (“plain, valley”), morn (“dark, black”) + (n-)gûl (“magic, necromancy”) In L:427, Tolkien explains that … “the triconsonantal group (rng) then being reduced to rg”.
Sindarin
imlad morgul
place name. Morgul Vale
Imlad Morgul
noun. valley of black magic
Sindarin name of Morgul Vale (LotR/694), a combination of imlad “valley” and morgul “sorcery” (RC/482).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name first appeared as N. Imlad Morghul (WR/223, note #25).