"stygian" in the adjective sense
"Hence loathed Melancholy.../In Stygian cave forlorn"- Milton
2. Acheronian, Acherontic, Stygian
dark 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
Source: WordNet® (An amazing lexical database of English)
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