Whenever you cast your second spell each turn, choose target creature an opponent controls. If it's tapped, put a stun counter on it. Otherwise, tap it. (If a permanent with a stun counter would become untapped, remove one from it instead.)
Spells that were cast before Shackle Slinger entered the battlefield count. If Shackle Slinger was the first spell you cast this turn, the next spell you cast this turn is your second spell.
