Mutilate’s -X/-X won’t apply to creatures that enter later in the same turn.

When a resolving effect modifies the characteristics of objects on the battlefield (such as power/toughness), the permanents that will be affected are “locked in” as the spell/ability resolves. So when you cast , it applies -X/-X to all creatures currently on the battlefield as the spell resolves, and that effect will last until the end […]

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Undying creatures and Black Sun’s Zenith.

Undying is a triggered ability that says “When this creature dies, if it didn’t have any +1/+1 counters on it, return it to the battlefield with a +1/+1 counter.” Most players know that +1/+1 counters and -1/-1 counters will cancel each other out if they’re on the same creature. This is a State-Based Action (“SBA”). […]

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