Chthonian is an adjective that means related to or resembling something from the underworld. 

Coming from the Greek word for ‘earth,’ our word of the day has been with us since the late 19th century, usually as a reference to the goings on in Greek mythology, for example:

As a kid, I loved Greek mythology, but I didn’t want the stories to get too chthonian. Any reference to the Ancient underworld would scare me away from Greek tales for weeks.