The Collaborative International Dictionary of English v.0.48:

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.

   Syn: chthonic, lower, nether.
        [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

chthonian
    adj 1: dwelling beneath the surface of the earth; "nether
           regions" [syn: chthonian, chthonic, nether]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "chthonian":
   Acherontic, Druid, Hadean, Lethean, Parsi, Plutonian, Plutonic,
   Sabaist, Satan worshiper, Satanist, Stygian, Tartarean,
   Zoroastrian, animal worshiper, anthropolater, arborolater,
   archaeolater, bibliolatrous, chthonic, demon worshiper, demoniast,
   demonist, demonolater, demonologer, demonologist, demonomist,
   dendrolater, devil worshiper, devilish, diabolist, fetishist,
   fetishistic, fire worshiper, heathen, hellborn, hellish,
   idol worshiper, idol worshiping, idolater, idolatress, idolatric,
   idolatrical, idolatrizer, idolatrous, idolist, idolistic, idolizer,
   infernal, nature worshiper, ophiolater, pagan, pandemoniac,
   pandemonic, phallic worshiper, plutonian, plutonic, purgatorial,
   pyrolater, snake worshiper, sorcerer, star worshiper, sulfurous,
   sun worshiper, theriolater, therolater, tree worshiper, zoolater,
   zoolatrous