Wordnet 3.0
ADJECTIVE (1)
1. 
 dark and dismal as of the rivers Acheron and Styx in Hades; 
- Example: "in the depths of an Acheronian forest"- Example: "upon those roseate lips a Stygian hue"-Wordsworth[syn: Acheronian, Acherontic, Stygian]
WordNet (r) 3.0 (2006):
Acheronian
    adj 1: 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]