Flatten

The price is the whole problem. Black has always sold this exact effect cheaper: Doom Blade lands a kill at two, and the long line of toughness-reducers (Last Gasp, Tragic Slip, Disfigure) shrinks bodies earlier on the curve. Paying for -4/-4 at instant speed is a rate no aggressive deck wants. What the extra mana buys is breadth of kill: subtracting toughness rather than destroying answers indestructible threats, regenerators, and anything else that would shrug off a targeted "destroy." Holding it through a combat step lets you shrink an attacker mid-swing to blow up a block or erase a pump. This toughness-reduction school of removal trades the cleanliness of destruction for immunity to the keywords that protect a body, and the tax for that flexibility is paid entirely in mana. It is filler-grade by intent, a common-rarity safety valve for a color that already carries sharper tools at every other point on the curve.