fiend vs demoniac

fiend and demoniac both are nouns.

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

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

fiend (noun)	demoniac (noun)
a cruel wicked and inhuman person	someone who acts as if possessed by a demon
a person motivated by irrational enthusiasm (as for a cause)
an evil supernatural being

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