
1) Chthonic 2) Cthulhu Mythos species 3) Infernal 4) Lower 5) Nether 6) Of the underworld 
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https://www.crosswordclues.com/clue/chthonian

1) Chthonic 
Found on 
https://www.crosswordclues.com/clue/chthonian
 [Cthulhu Mythos]
[Cthulhu Mythos]  Chthonians (z; from Greek: chthon, `earth`) are fictional creatures in the Cthulhu Mythos. The species is the creation of Brian Lumley and was first featured in his short story `Cement Surroundings` (1969)—though the creature never made a direct appearance. The chthonians had a more prominent role in Lumley`s novel The Bu...
Found on 
http://en.wikipedia.org/wiki/Chthonian_(Cthulhu_Mythos)

of, like or pertaining to the earth or the underworld
Found on 
http://phrontistery.info/c.html

• (a.) 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 deitie...
Found on 
http://thinkexist.com/dictionary/meaning/chthonian/
 Chtho'ni·an  adjective
Chtho'ni·an  adjective [ Greek ... in or under the earth, from ..., ..., 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 characteris...
Found on 
http://www.encyclo.co.uk/webster/C/75

[
adj] - of the underworld
Found on 
http://www.webdictionary.co.uk/definition.php?query=chthonian

chthonian 1. Referring to, or pertaining to, the deities, spirits, and other beings dwelling under the earth.  2. Relating to the underworld.  3. Dwelling in, or beneath, the surface of the earth.
Found on 
http://www.wordinfo.info/words/index/info/view_unit/467/2
 chthonic adjective
chthonic adjective dwelling beneath the surface of the earth; `nether regions`
Found on 
https://www.encyclo.co.uk/local/20974

of or pertaining to the deities, spirits, and other beings dwelling under the earth.
Found on 
https://www.infoplease.com/dictionary/chthonian
  No exact match found.