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.

of the underworld; "nether regions"

A chthonian is a fictional character in the Cthulhu Mythos. The being is the creation of Brian Lumley and was first featured in his short story "Cement Surroundings" (1969)—though the creature never made a direct appearance. The chthonians had a more prominent role in Lumley's novel The Burrowers Beneath (1974).