Chthonian is a word that describes something related to the underworld or the earth. Antonyms for most chthonian could include words such as heavenly, celestial, or ethereal, which describe things associated with the sky or heaven. Other antonyms could include words like exalted, angelic, divine, or spiritual, all of which describe things of a higher or more elevated nature. Some antonyms that are more specifically related to the idea of the underworld could include words like bright, radiant, or shining, all of which suggest a sense of light and clarity that is the opposite of the darkness and obscurity associated with chthonic things.