Elana, Frenzied Stalwart

Flying, lifelink, hexproof

{2}: Exile the top card of your library, until end of turn, you may cast that card. Elana, Frenzied Stalwart gets +X/+0 where X is the mana value of that card.

At the beginning of your end step, sacrifice Elana, Frenzied Stalwart if its power is greater than 13, it deals 13 damage to you, return it to the battlefield under its
owner's control at the beginning of your next upkeep.