Of the underworld or the depths of the earth; earthy in the sense of subterranean, primeval, and uncanny (literary; mythological). Chthonian implies ancient powers below the surface, fertility and dread braided together.

From Greek khthṓn “earth” (especially the earth as the world below), yielding khthónios “of the underworld.” In English the term keeps its mythic gravity: earth not as soil for gardens, but as the realm that swallows and remembers.