What does "chthonian" mean?

Chthonian: Designating, or pertaining to, gods or spirits of the underworld; esp., relating to the underworld gods of the Greeks, whose worship is widely considered as more primitive in form than that of the Olympian gods. The characteristics of chthonian worship are propitiatory and magical rites and generalized or euphemistic names of the deities, which are supposed to have been primarily ghosts.