Torment of Hailfire
Each iteration hands every opponent the same trilemma, and every branch of it costs them something: bleed three life, pitch a card, or sacrifice a nonland permanent of their own choosing. Run the loop X times and the spell taxes three axes at once (life, hand, board), pressuring whichever resource the victim is trying to hoard. A pure life-loss effect can be raced; a pure discard is dead against an empty hand; a sacrifice clause is shrugged off by a resilient board. Bundling all three and letting the opponent pick the poison is what closes those escape routes: there is no single state of the game that turns any single iteration off. The cost structure is the catch. Only the two black pips are fixed; the loop runs entirely on generic mana, so any mana at all feeds X, and the spell is inert at small X and unanswerable at large X. That places it closer to a payoff than to removal: the reward a deck banks a mana lead toward, then converts into one crushing turn. The slow, deliberate march (opponent by opponent, permanent by permanent, life point by life point) is its signature, and it is why a resolved copy with enough mana behind it tends to end the game where it sits.
