Acheronian vs Acherontic

Acheronian and Acherontic both are adjectives.

Word	Noun	Adjective	Verb	Adverb
Acheronian	No	Yes	No	No
Acherontic	No	Yes	No	No

As adjectives, Acheronian and Acherontic are synonyms defined as:

Acheronian and Acherontic: dark and dismal as of the rivers Acheron and Styx in Hades

Other synonyms of Acheronian include Stygian.

Acheronian (adjective)	Acherontic (adjective)
dark and dismal as of the rivers Acheron and Styx in Hades	dark and dismal as of the rivers Acheron and Styx in Hades