Doom Blade

The "nonblack" clause is the whole negotiation. Cheap unconditional removal had always been black's birthright, but pricing it at two mana with no downside would have made the color's answer suite trivially good, so the design pays for the rate with a color tax: black creatures walk free. That single word turns a clean kill spell into a slightly awkward one, since the mirror match is exactly where you most want to be killing things and exactly where this whispers nothing. It also quietly props up black-creature decks against their own color's best removal, a self-balancing wrinkle that keeps black from being strictly the best at murdering everything. The template proved durable enough to spawn a small family of two-mana conditional kills, each carving the exclusion differently: Go for the Throat spares artifacts instead of black, Ultimate Price hits only monocolored creatures, Cast Down passes over legends. Doom Blade is the original cut of that cloth, the version where the carve-out is the creature's color rather than its type or supertype. It is the reference point those later spells were drawn against, the proof that you could give black a Terror-style instant without a self-inflicted life loss as long as you handed the opponent one safe corner to hide in.