The Collaborative International Dictionary of English v.0.48:
Acherontic \Ach`e*ron"tic\, a. Of or pertaining to Acheron; infernal; hence, dismal, gloomy; moribund. [1913 Webster]WordNet (r) 3.0 (2006):
Acherontic 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]