Chthonic

ˈθɒnɪk

adjective

relating to or inhabiting the underworld

The term 'chthonic' comes from Greek mythology, where it referred to deities or spirits that dwelled within the earth. It is often used to describe things that are associated with the underworld or the earth's depths.

Then one day, we woke up to find this chthonic spirit turned political, and shook our heads with sorrow over its ungrateful nationalism—truly the fine flower of our innocent efforts.

T. E. Lawrence

Seven Pillars of Wisdom