Cut Short

The gate on this removal is a piece of positional discipline: it can only touch a planeswalker that was activated this turn or a creature that has already tapped. Both restrictions do the same structural work, pointing the spell at permanents that have committed to acting. A planeswalker is fair game only after it has spent its loyalty for the turn; a creature is fair game only once it has tapped for any reason, meaning the threat has already been announced before you answer it. That reactive framing is what lets white have destruction on an instant across two card types at once, and it neatly excludes the summoning-sick, untapped blocker sitting back on defense. Convoke pays for the rest: the board can chip in for the generic cost, and white creatures can even cover the white pip, so the mana investment collapses in exactly the decks that flood the battlefield with bodies and then want an efficient answer. The timing is the sharpest part of the design. Cast in response to a planeswalker's activated ability, it kills the walker but leaves the ability on the stack to resolve, so an opponent who spends loyalty gets the effect and loses the permanent anyway. Against creatures, the tapped clause turns it into an attacker-punisher: it waits until a swing has been declared and the attacker turned sideways, then destroys it once the opponent has committed to an attack they can no longer take back.