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Target player creates X 2/2 black Zombie creature tokens, then up to one target creature gets -1/-1 until end of turn for each Zombie that player controls.
If the targeted creature becomes an illegal target, the targeted player will still get Zombie tokens.
You choose up to one target creature total, not a separate target for each Zombie that player controls.
You may cast Dark Salvation with X as 0 just to have a creature get -1/-1 for each Zombie a player controls, and you may cast it without targeting any creature just to have a player get Zombie tokens. If you really just want to cast a spell, you can even cast it with X as 0 without a target creature and have neither effect do anything.
