infernal – being of the underworld; “infernal regions”
2.
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
dark – devoid of or deficient in light or brightness; shadowed or black; “sitting in a dark corner”; “a dark day”; “dark shadows”; “dark as the inside of a black cat”