As an additional cost to cast this spell, exile a creature card from your graveyard.
Corpse Explosion deals damage equal to the exiled card's power to each creature and each planeswalker.
Once you have begun to cast Corpse Explosion, there is no point at which players can respond by removing cards from your graveyard to prevent you from being able to pay for it with a specific creature card.
Use the card's power as it last existed in the graveyard to determine how much damage is dealt.