Something chthonic dwells beneath the earth. This word usually refers to mythological creatures.
Every heard about the mole men, who live underneath the ground in tunnels? Or the mutants who live in the sewers? Or even the Fraggles? Of course, those critters are imaginary, but they're examples of chthonic creatures: beings who live under the surface of the earth. Chthonic beasts are more likely to be demons than angels, so this adjective has a hellish component to it. Many mythologies feature chthonic creatures.