demon vs demoniac

demon and demoniac both are nouns.

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

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

demon (noun)	demoniac (noun)
an evil supernatural being	someone who acts as if possessed by a demon
a cruel wicked and inhuman person
someone extremely diligent or skillful

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