Source : Webster's Revised Unabridged Dictionary (1913)

Chthonian \Chtho"ni*an\, a. [Gr. ? in or under the earth, fr. ?,
   ?, earth.]
   Designating, or pertaining to, gods or spirits of the
   underworld; esp., relating to the underworld gods of the
   Greeks, whose worship is widely considered as more primitive
   in form than that of the Olympian gods. The characteristics
   of chthonian worship are propitiatory and magical rites and
   generalized or euphemistic names of the deities, which are
   supposed to have been primarily ghosts.

Source : WordNet®

chthonian
     adj : of the underworld; "nether regions" [syn: {chthonic}, {lower},
            {nether}]