Voldaren Bloodcaster // Bloodbat Summoner [Innistrad: Crimson Vow Prerelease Promos]

Defender
At the beginning of combat on your turn, you may clash with an opponent. If you win, Sentry Oak gets +2/+0 and loses defender until end of turn. (Each clashing player reveals the top card of their library, then puts that card on the top or bottom. A player wins if their card had a higher mana value.)