A pestal deck should have no problem paying either 18 (Nightfall) or 24 (Eclipse)
quanta, even assuming some majickal situation wherein the user of said pestal had all 6 copies of said card in their hand at the same time.
I'm a big fan of Darkness, but I don't think it needs this buff. If Nightfall|Eclipse WERE to stack though, it would certainly warrant a cost increase. Doing that could even be detrimental to some decks.