The Collaborative International Dictionary of English v.0.48:

Stygian \Styg"i*an\ (st[i^]j"[i^]*an), a. [L. Stygius, fr. Styx,
   Stygis, Gr. Sty`x, Stygo`s, the Styx.]
   Of or pertaining to the river Styx; hence, hellish; infernal.
   See Styx.
   [1913 Webster]

         At that so sudden blaze, the Stygian throng
         Bent their aspect.                       --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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, Stygian]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "Stygian":
   Acheronian, Acherontic, Cimmerian, Lethean, Plutonian, Plutonic,
   Tartarean, chthonian, chthonic, clouded, cloudy, dark and gloomy,
   devilish, funereal, gloomful, glooming, gloomy, hellborn, hellish,
   ill-lighted, ill-lit, infernal, lowering, overcast, pandemoniac,
   pandemonic, purgatorial, somber, sombrous, stormy, sulfurous