Just one thing I'd like to point out:
"All
other death or darkness creatures..."
The highlighted part to me implies that the actual Bone Spirit
doesn't receive the 'Nightfall'-esque bonus when it's out on the field, which also doesn't stack with the Nightfall effect either. This means that Bone Spirit is a terrible creature to spam as an individual component, BUT it serves as an attacking field permanent (if Nightfall had stats). With that said, I could see you reducing the cost of both cards by at least 1 (so it's 7
-> 6
), and also clarifying within your notes that the Bone Spirit doesn't benefit from its own ability.
The Momentum throws me off a bit, but that doesn't matter too much. Deathstalkers would benefit the most from this card in mono-Death decks, but aside from that, it's a good concept.