Rebuke

The discount is paid for in initiative. Unconditional creature destruction in white usually costs more or arrives wrapped in a clause about converted mana value, tapped status, or color; here the rate drops to three mana flat because the spell can only fire once a creature has committed to combat. That restriction does real strategic work. It cannot answer a problem on your own turn, cannot break up a combo before it assembles, and cannot pick off a quiet value engine that never swings. What it can do is punish the attack itself: a one-for-one kill at instant speed, with the upside that the opponent has already chosen the target for you and may have sunk a pump spell or combat trick into the swing, turning the trade lopsided. White has a long tradition of this conditional-kill design, trading the freedom of sorcery-speed Wrath effects for instant-speed answers that ask the opponent to walk into them. The card sits in that lineage as a defender's instant: cheap insurance for the player who would rather hold up mana than tap out, and a tax on aggression that rewards a patient, reactive posture over a proactive one.