demonism vs necromancy

demonism and necromancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
demonism	Yes	No	No	No
necromancy	Yes	No	No	No

As nouns, necromancy is a hypernym of demonism; that is, necromancy is a word with a broader meaning than demonism:

demonism: a belief in and reverence for devils (especially Satan)
necromancy: the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world

Other hypernyms of demonism include black art, black magic, sorcery.

demonism (noun)	necromancy (noun)
a belief in and reverence for devils (especially Satan)	conjuring up the dead, especially for prophesying
	the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world