Dread Shade

The lineage of black mana sinks runs straight through this design: a creature whose only trick is converting swamps into stats, one mana at a time, with no ceiling but the black sources you can untap. The Shade pattern before it (Nantuko Shade, and the pumpable bodies going back to the earliest sets) asked the same question, but the triple-black casting cost here pushes the commitment further, demanding a mono-black manabase before the card does anything at all. That cost is the bargain, and the payoff is a body that scales linearly with your open mana, capable of ending a game in one attack step if you have held enough sources back. Because the pump is tap-free and activated, it works at instant speed in any phase: you commit black in response to blockers to punch through, or in response to a damage-based burn spell to survive the point of damage that would otherwise kill you. The catch that keeps it from being a permanent threat is the "until end of turn" clause; the growth evaporates when the turn ends, so this is not a creature you build up over several turns. It is a creature you spend everything on once, in a single decisive window. In a game black is trying to close quickly, that all-at-once burst was always the point, and permanence would only have slowed the payoff down.