Acheronian is an adjective used to describe something that is dark, gloomy, or murky. If you want to convey the opposite of acheronian, you can use antonyms like bright, sunny, cheerful, or optimistic. These words indicate positivity, hopefulness, and a sense of happiness or joy. Other antonyms for acheronian include luminous, sparkling, brilliant, radiant, and dazzling, which all describe things that are well-lit, shining, and bright. Using these antonyms can help you express a range of emotions and create a contrast with the darkness and heaviness of the term acheronian.