I like the idea of leveling the playing field by giving reasons to play less used brigades, but I think the answer is not just to print cards in those brigades but also require them to be used by entirely separate deck types and not just all thrown in a splash deck.
Demonic Heresy:
This one IMO has the most potential to just make splash decks more powerful instead of buffing heretics or demons. It's basically just a second confusion that splash decks get to run. I do think there should be a card with these brigades but I think it should also require you to be playing a certain strategy. This card seems to be aimed at heretic/demon decks and the cards that currently support that deck type focus on having humans and heretics in battle at the same time. Because of that, I think it should have an effect that first requires you to be playing heretics and demons and secondly reward you the further into that strategy you are. For example, "If used by a human, opponent must discard a card from hand or battle for each demon in battle. If used by a demon, band to up to X heretics and protect a NT lost soul from rescue for each one banded in this way (X=opponents good brigades in battle)". The ability is still very powerful, but only if your deck is built with it specifically in mind.
Spiritual Death:
This card has a unique, powerful affect and does not buff the top brigades or most splash decks so I like it a lot already. The only thing I would change is putting the used by requirement on the main ability itself instead of only making it cbp. This forces it entirely into a specific strategy and lowers the risk it aides in accidently making some future decks similar to today's crimson/brown/pgreen slash decks.
Slaughtering Blade:
This card is great in that it already pretty much requires it to be used in a certain strategy. The base effect is not good, in fact a card already exists that is arguably straight up better. This card becomes powerful when put in a specific deck and that deck is underused right now to boot. I like this card a lot.