Acherontic vs Acheronian

Acherontic and Acheronian both are adjectives.

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

As adjectives, Acherontic and Acheronian are synonyms defined as:

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

Other synonyms of Acherontic include Stygian.

Acherontic (adjective)	Acheronian (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