Fall of the Hammer

• WWizards of the Coast2014-02-01
  Official Ruling
  The amount of damage dealt is based on the first target creature’s power as Fall of the Hammer resolves.
• WWizards of the Coast2014-02-01
  Official Ruling
  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.

Rulings via Scryfall