Source : Webster's Revised Unabridged Dictionary (1913)

Stygian \Styg"i*an\, a. [L. Stygius, fr. Styx, Stygis, Gr. ?, ?,
   the Styx.]
   Of or pertaining to the river Styx; hence, hellish; infernal.
   See {Styx}.

         At that so sudden blaze, the Stygian throng Bent their
         aspect.                                  --Milton.

Source : WordNet®

Stygian
     adj 1: hellish; "Hence loathed Melancholy.../In Stygian cave
            forlorn"- Milton
     2: 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 [syn: {Acheronian},
         {Acherontic}]