Recurring Nightmare
The engine that taught a generation of players what a recursion loop actually feels like. The trick is in the cost: returning the enchantment to hand instead of tapping or sacrificing it permanently means the only resource you ever spend is the creature you feed it, and you get to replay the enchantment next turn for three mana. Pair that with any creature whose death or arrival generates value (a sacrifice payoff, an enters-the-battlefield trigger, a token producer) and the same body cycles in and out indefinitely, the enchantment shuttling between hand and battlefield as a permanent fixture. The sorcery-speed restriction is the only brake, and it is a soft one: it stops you from doing this at instant speed during combat or in response to removal, but it does nothing to cap how many times you reload across your own turns. What makes the design hold together is that it never reads cards from your library or hand; it eats the graveyard, so the deck has to manufacture both the fodder and the reanimation targets, and the payoff scales with how degenerate that synergy is. That ceiling is high enough that the card has carried a banned or restricted name in older constructed formats for most of its existence, a status that says more about the loop than any single combo it enables.
