Sagittars' Volley

Each creature with flying controlled by any of your opponents is dealt damage, even if that opponent didn’t control the target creature with flying.

If the target creature is an illegal target by the time Sagittars’ Volley tries to resolve, the spell doesn’t resolve. No creatures are dealt damage. If the target is legal but not destroyed (most likely because it has indestructible), creatures with flying your opponents control are dealt damage, possibly including the target creature.