Nightshade Assassin

The removal scales with how committed to black you already are: reveal X black cards from your hand, hand the targeted creature -X/-X, and pay nothing extra to do it. Crucially, revealing is not discarding, so the cards that set the price stay in hand to be cast later. The assassin reads how black your hand actually is and prices the kill off it without spending a single one of the cards doing the pricing. Madness closes the loop. A discard outlet (the kind that asks you to pitch cards as a cost) turns a four-mana enters-the-battlefield trigger into a two-mana instant-speed ambush for : the same -X/-X effect, fired on the opponent's turn after they have over-committed to a swing or a block. The two costs draw on different resources, which is the elegant part: the mana pays the madness discount, while the cards in hand only need to be revealed, so a black-flooded hand both finances the cast and stocks the kill. First strike on a 2/1 is a small dividend once the trigger resolves. This is early-era black removal built around a single question, how much black are you really playing, and it lets that hand answer the question twice without ever leaving it.