Target creature you control deals damage equal to its power to another target creature.
As Fall of the Hammer tries to resolve, if only one of the targets is legal, Fall of the Hammer will still resolve but will have no effect: If the first target creature is illegal, it can’t deal damage to anything. If the second target creature is illegal, it can’t be dealt damage.
The amount of damage dealt is based on the first target creature’s power as Fall of the Hammer resolves.