Crovax the Cursed

A creature whose body shrinks every turn you don't feed it, and that fragile contract is the entire design. The upkeep clause builds a closed economy: each turn you either sacrifice a creature to grow him or watch a counter peel off, so the card is only as durable as the board you're willing to throw under it. That makes it a sacrifice engine and a payoff in the same slot, a piece that converts spare creatures into a body that keeps growing past the point most four-drops cap out. The flavor and the function line up neatly: this is the Crovax of the Weatherlight saga at his cursed turn, the card literally feeding on the lives around him to stay strong. The flying activation is the closer, a cheap repeatable evasion grant that turns an oversized ground body into a clock once the counters have stacked. The friction worth naming is the downside half of the trigger: with an empty board you can't sustain him, and an opponent who clears your fodder also starts the slow countdown on Crovax himself. That self-decay is the cost the rate pays for, because a creature that only ever grows would be a different (and much safer) card. The structure (a single permanent that is both the sacrifice outlet and the reward, rather than two cards split across an outlet and a payoff) was an early take on black's sacrifice engines, and variations on it have surfaced repeatedly since.