Snapdax, Apex of the Hunt

Mutate {2}{B/R}{W}{W} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Double strike
Whenever this creature mutates, it deals 4 damage to target creature or planeswalker an opponent controls and you gain 4 life.

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If there is no legal target for Snapdax’s triggered ability when it triggers, it won’t be put onto the stack at all and you won’t gain 4 life. Similarly, if the target creature or planeswalker is an illegal target by the time the ability tries to resolve, the ability won’t resolve and you won’t gain 4 life.