(adj) dark and dismal as of the rivers Acheron and Styx in Hades; ; -Wordsworth, Syn. Acherontic, Stygian, Example:in the depths of an Acheronian forest; upon those roseate lips a Stygian hue
The Collaborative International Dictionary of English (GCIDE) v.0.53 Collaborative International Dictionary (GCIDE)
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 ]