Heretic's Punishment

• WWizards of the Coast2011-09-22
  Official Ruling
  The mana value of a double-faced card in your graveyard is the mana value of the front face.
• WWizards of the Coast2011-09-22
  Official Ruling
  If you have two or fewer cards in your library when the ability resolves, all of them will be put into your graveyard. Heretic's Punishment will still deal damage equal to the highest mana value among those cards.
• WWizards of the Coast2011-09-22
  Official Ruling
  If all three cards have a mana value of 0, no damage will be dealt.
• WWizards of the Coast2011-09-22
  Official Ruling
  If the targeted permanent or player is an illegal target by the time the ability resolves, the entire ability won't resolve. No cards will be put into your graveyard, and no damage will be dealt.

Rulings via Scryfall