The word "Chthonic" derives from the Greek χθόνιος (khthonios), which means 'in or under the earth/soil', and is used in connection with gods or spirits of the underworld.

In Greek religion and mythology, the Chthonic tradition is much older than the Olympian one. 'Chthonic' tends to have rather dark connotations now days (no little thanks to H P Lovecraft!), but the Greeks regarded the chthonic deities as an earlier, more primitive complement to the 'modern' Olympian deities, and were mostly associated with the more primeval and basic aspects of life - fertility, death, illnesses, magic, childbirth, crops etc.

Since Chthonic deities were believed to dwell in the earth or beneath it, sacrificial offerings were made into the earth itself, by pouring liquids onto the earth, throwing beasts into an underground pit, or burying them. Offerings were not usually cooked and shared among the worshippers as with ritual sacrifices made to the Olympian pantheon.

Hades, Persphone and the Erinys/Eumenides are generally considered the principle chthonic deities. However, many other Greek deities, including Hermes, Hekate, Demeter, Zeus, Dionysos and Gaia, had both Olympian and Chthonic aspects and include amongst their titles the epithet 'Khthonios' or 'Chthonia' when that function is implied.

The main source for the myths surrounding the chthonic deities is Hesiod's Theogony and Works and Days - although he wrote much later than Homer's depictions of the Olympic deities he was describing myths from an older tradition.