(sometimes used attributively) An area of peril, darkness, or evil, which people fear to visit or explore.
From Mordor, a bleak realm ruled by the dark lord Sauron, in J. R. R. Tolkien's fictional Middle-earth. Tolkien created the name in his constructed language Sindarin, from mor (“dark, black”) and dor (“land”).
English Wiktionary. Available under CC-BY-SA license.