Rivanna, Goddess of Mana

Rivanna's power and toughness are each equal to
the number of cards in your graveyard and exile.
If Rivanna is in play or your graveyard, your
nonpermanent spells without {x} can be paid
by exiling cards from your hand. Each card you
exile this way pays for any 3 mana.
You may cast Rivanna from graveyard and exile.
If you do, exile 2 cards from your hand as an
additional cost.