When Blood Spatter Analysis enters the battlefield, it deals 3 damage to target creature an opponent controls. Whenever one or more creatures die, mill (To mill a card, a player puts the top card of their library into their graveyard.) a card and put a bloodstain counter on Blood Spatter Analysis. Then sacrifice it if it has five or more bloodstain counters on it. When you do, return target creature card from your graveyard to your hand.