Adjectives

(s)Stygianhellish“Hence loathed Melancholy.../In Stygian cave forlorn” (Milton)
(s)Acheronian, Acherontic, Stygiandark and dismal as of the rivers Acheron and Styx in Hades“in the depths of an Acheronian forest”, “upon those roseate lips a Stygian hue” (Wordsworth)