Rivanna, Goddess of the Veil

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, each
mana you pay when casting a spell can be paid by
exiling a card from your graveyard and paying 1
life instead.
You may play Rivanna from graveyard and exile.
If you do, sacrifice 2 nontoken permanents as an
additional cost.