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Noxious Hatchling enters the battlefield with four -1/-1 counters on it.
Wither (This deals damage to creatures in the form of -1/-1 counters.)
Whenever you cast a black spell, remove a -1/-1 counter from Noxious Hatchling.
Whenever you cast a green spell, remove a -1/-1 counter from Noxious Hatchling.
If there are no -1/-1 counters on it when the triggered ability resolves, the ability does nothing. There is no penalty for not being able to remove a counter.
If you cast a spell that’s both of the listed colors, both abilities will trigger. You’ll remove a total of two -1/-1 counters from the Hatchling.
