Swarm Shambler
This creature enters with a +1/+1 counter on it.
Whenever a creature you control with a +1/+1 counter on it becomes the target of a spell an opponent controls, create a 1/1 green Insect creature token.
, 
: Put a +1/+1 counter on this creature.
Illustrated by Nicholas Gregory
This printing
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If a spell targets a creature you control with a +1/+1 counter on it more than once, Swarm Shambler’s ability triggers only once.
An ability that triggers when a creature becomes the target of a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
You create just one Insect token, no matter how many +1/+1 counters the target creature has on it.
If a creature without a +1/+1 counter on it becomes the target of a spell an opponent controls, putting a +1/+1 counter on it after that won’t cause Swarm Shambler’s middle ability to trigger.
