unpleasantly dark, and making you feel nervous or afraid (stygius, from , from Styx river in ancient Greek stories which people cross over when they die)
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
[ 'sti-j(E-)&n ] (adjective.) 1566. From Latin stygius, from Ancient Greek Στύγιος (Stugios, “relating to Styx”), from Στύξ (Stux, “Styx, chief river of underworld”).