Cobalt, Heart of the Cards

Whenever you cast a spell without spending
mana, put a +1/+1 counter on Cobalt.
{4}: Target opponent chooses a number, then you
reveal the top card of your library. If it's a
nonland card with that converted mana cost,
exile it and it deals that much damage to you or
a creature you control of that opponent's choice.
Otherwise, you may cast it without paying its
mana cost. Any player may activate this
ability.