Einswid, Petal of Nightshade

Deathtouch
Sovereign -- Creatures you control have "Whenever this creature deals combat damage to a player, that player reveals a card at random from his or her hand".
Whenever a player reveals a card from his of her hand, you may pay {b}{u}. If you do, you may exchange the revealed card with any non-land card in your hand. You lose 1 life. (You put the revealed card in your hand, and the opponent puts your chosen card in their hand.)