Nightshade Seer

The whole design hinges on a tax you pay in cards you already wanted to keep. The activation shrinks a target by however many black cards you reveal, but those cards stay in hand: you are not discarding them, just showing them, which means a mono-black hand can throw a -3/-3 or -4/-4 at a creature without spending a single card beyond the mana. That is the clever part of the constraint. The body is a fragile 1/1 and the ability requires tapping, so summoning sickness keeps it off the table the turn it lands; this never reads as a removal engine you cast and immediately fire. But a Seer left alive in a deck stuffed with black cards turns repeatable, scalable creature kill into a function of how committed you are to the color. It rewards going deep on black rather than splashing it, since every off-color card in hand is a card that cannot feed the ability. The effect is a toughness shrink rather than a destroy, which is precisely what makes it dangerous: drop a creature to zero or less toughness and it simply ceases to exist, no destruction event for indestructibility to ignore and no damage for regeneration to soak. Against the fat blockers of its era, a single activation with enough black cards behind it usually meant a dead creature and a Wizard standing where it used to be.