Stygian

adjectives

hellish
"Hence loathed Melancholy.../In Stygian cave forlorn"- Milton
is similar to
being of the underworld
"infernal regions"
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