Arc Lightning

• WWizards of the Coast2014-09-20
  Official Ruling
  You choose how many targets Arc Lightning has and how the damage is divided as you cast the spell. Each target must receive at least 1 damage.
• WWizards of the Coast2014-09-20
  Official Ruling
  If some of the creatures are illegal targets as Arc Lightning tries to resolve, the original division of damage still applies but no damage is dealt to the illegal targets. If all targets are illegal, Arc Lightning doesn’t resolve.

Rulings via Scryfall