Swarm Shambler
Creature — Fungus Beast
Swarm Shambler enters the battlefield 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 Swarm Shambler.
Illustrated by
Zendikar Rising#207RareEnglishLegality
Rulings
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.
You create just one Insect token, no matter how many +1/+1 counters the target creature has on it.
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.
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.