Scar

Reducing a creature's toughness to zero is a different kind of kill than burning it down, and this one-mana instant is built entirely on that difference. Damage fades after combat and has to overcome a single toughness number all at once; a permanent counter shrinks the creature for good and keeps company with every other counter the board has accumulated. Against a creature already wearing -1/-1 counters it pushes them closer to death; against an X/1 it kills outright; against anything bigger it leaves behind a body that stays diminished into future turns. Because toughness at zero triggers state-based death rather than dealing damage, it slips past the regeneration shields and prevention effects that blunt burn. The cost being castable by either black or red is the part doing quiet work: two colors that both crave cheap interaction can share the slot without either claiming it, a flexibility suited to a setting whose mana bends to whatever surrounds it. What it surrenders is reach and reach to the face: it cannot touch a player, and against a creature too large to drag to zero it only makes the thing permanently smaller. This is cheap toughness-reduction removal in its plainest form, trading the speed and flexibility of a damage spell for permanence and the upside of leaving a counter behind for whatever cares to use it.