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Wildwood Scourge enters the battlefield with X +1/+1 counters on it.
Whenever one or more +1/+1 counters are put on another non-Hydra creature you control, put a +1/+1 counter on Wildwood Scourge.
Abilities that trigger when counters are put on a creature trigger when a creature enters the battlefield with counters and when a player puts counters on a creature.
If you have two non-Hydra creatures with Wildwood Scourge's ability (perhaps because they were mutated using the Ikoria: Lair of Behemoths mutate ability), putting a +1/+1 counter on any non-Hydra creature you control will cause the two abilities to trigger each other indefinitely. If no player takes an action to break this loop, the game ends in a draw.
Wildwood Scourge gets just one +1/+1 counter, no matter how many counters were put on the non-Hydra creature.
