Roil's Retribution

• WWizards of the Coast2015-08-25
  Official Ruling
  You choose how many targets Roil’s Retribution has and how the damage is divided as you cast the spell. Each target must receive at least 1 damage.
• WWizards of the Coast2015-08-25
  Official Ruling
  If some, but not all, of the targets become illegal by the time Roil’s Retribution tries to resolve, the original division of damage still applies, but no damage is dealt to illegal targets. If all the targets are illegal, the spell won’t resolve.

Rulings via Scryfall