Dark Confidant

The card-advantage engine that made you pay for it, and the price scaled with the card. Black has always bought cards with life: Phyrexian Arena drips a steady one per turn, Necropotence converts life directly into hand size. What set this design apart is the variance. The life loss is whatever the next card's mana value happens to be, which means the same engine that hands a lean, low-curve deck a near-free card every turn turns vicious the moment the top of the library is a five-drop bomb or an expensive planeswalker. The deckbuilder who wants the engine has to keep the curve honest, because the card punishes greedy top-ends in exactly the way it rewards disciplined ones. The 2/1 body folds that bargain into the math too: it dies to almost anything, so the engine isn't guaranteed to run long, and the format conversation around it became a referendum on how cheap and disciplined a black deck could afford to be. For roughly a decade it defined an entire approach to fair black midrange and aggro, a creature whose drawback you tune your other forty cards to minimize. Few designs have extracted as much deckbuilding discipline from a single self-inflicted cost, and fewer still have stayed relevant in eternal formats while asking their pilot to do real work to make the engine safe.