Knockout Blow

A combat trick that adjusts its own price by inspecting the target's color, and the discount is what everything else hangs on. Four damage plus two life at instant speed is a fair rate against most creatures, but the reduction is the real lever: point it at a red creature and the cost collapses to a single white mana, turning a serviceable answer into a rout against aggressive red decks that live and die in the attack step. That conditional pricing is white's oldest hostility toward red made explicit, part of a long sideboard tradition of hosers that simply cost less against a hated color. The tension the design resolves is the perennial problem of maindeckable hate: a card that only reads as anti-red would rot in most matchups, so the base mode stays generically useful (removal for a threat that has committed to combat, with a life swing that matters in a race) while the discount rewards you for facing the archetype it was built to punish. Restricting the target to attacking or blocking creatures is the wrinkle that separates it from open-ended spot removal: it demands a combat step, so it answers threats already on the board rather than pre-empting them, and it asks you to wait for a creature to attack or block before you can cast it at all.