As you cast this, you target a creature and an opponent. The opponent does not have to be that creature's controller.

If the opponent becomes an illegal target before the spell resolves (but the creature doesn't), the spell will still be able to access information about that opponent. It will resolve as normal and deal the appropriate amount of damage to the creature.

If the creature becomes an illegal target before the spell resolves (but the opponent doesn't), the spell will be unable to affect that creature. Although the spell will still resolve, it won't deal any damage.