Darksteel Reactor

An alternate win condition that costs nothing to advance and almost nothing to protect, which is exactly the design problem it courts. Twenty charge counters is twenty upkeeps of patience: a clock so slow it would be unplayable if interaction worked normally against it, so indestructibility plugs the obvious hole. You cannot Naturalize it away, cannot blow it up with the board, cannot answer it at sorcery speed in the colors that usually clean up artifacts. That leaves bounce, exile, and counters-removal as the real outs, and the card is built knowing most decks pack none of them. The honesty in the design is the twenty: a number large enough that a fair deck running this as its only plan loses to a normal aggressive curve well before the meter fills. It only becomes a threat when something else accelerates the counters, at which point the indestructible body turns "you have a fragile combo piece" into "you have a combo piece nobody can destroy with the cheap answers." Wizards has returned to the proliferate-and-counters axis many times since, but the original framing here is starker than its successors: no doubling, no payoff for partial progress, just a single binary that flips at twenty and a printed promise that the rock holding your win condition will not break. The whole card is a wager that you can build everything around it to survive the wait.