monster vs demoniac

monster and demoniac both are nouns.

Word	Noun	Adjective	Verb	Adverb
monster	Yes	No	No	No
demoniac	Yes	Yes	No	No

As nouns, demoniac is a hyponym of monster; that is, demoniac is a word with a more specific, narrower meaning than monster:

monster (noun)	demoniac (noun)
(medicine) a grossly malformed and usually nonviable fetus	someone who acts as if possessed by a demon
a cruel wicked and inhuman person
a person or animal that is markedly unusual or deformed
an imaginary creature usually having various human and animal parts
someone that is abnormally large and powerful

monster (adjective)	demoniac (adjective)
	of, pertaining to, or like a demon or possession by a demon