chthonic
Dwelling within the underworld.
The young pantheon had remanded their elders to the role of smouldering, chthonic gods; to inhabiting dark, deep places, hidden from mortal eyes and influence.
Where it comes from
From Ancient Greek χθών (khthṓn, “ground, soil”) + -ic.
Adapted from the Wiktionary entry chthonic (CC BY-SA 4.0).