The Greeks always had a word for it, and this time it's an ominous one. Chthonic (pronounced THAHN-ick, rather like the drive-in restaurant by someone with a lisp) means dark, primitive, mysterious, ominous, infernal. It derives from the Greek chthon, "earth." It reeks of the Underworld and often refers to the grim deities that inhabit it. Chthonian is another spelling.