demoniac vs monster

demoniac and monster both are nouns.

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

As nouns, monster is a hypernym of demoniac; that is, monster is a word with a broader meaning than demoniac:

Other hypernyms of demoniac include demon, devil, fiend, ogre.

demoniac (noun)	monster (noun)
someone who acts as if possessed by a demon	(medicine) a grossly malformed and usually nonviable fetus
	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

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