Spore Frog

A Fog stapled to a body, with the body itself being the cost. The conversion is the whole point: it turns a one-shot combat-damage prevention spell into a renewable one by surviving in the graveyard, where any way to return it hands you a fresh blank turn against an attacker. The crucial wrinkle is that the prevention is an activated ability whose cost is sacrificing the Frog itself; you cannot feed it to an outside sacrifice outlet and still buy the Fog, so the loop you actually want is reanimation, not a free sac engine. Pair it with a way to return it and you hold a recurring lock on the combat step, bending each turn around whoever controls the recursion. Its strict scope is what bounds the lock: it stops only combat damage, not burn, not life loss, not the board state that develops while the attack stalls. The Frog delays rather than answers, asking its controller to convert tempo into time and then spend that time building toward something the prevention cannot itself provide. Its presence at the table is less a creature than a standing condition: as long as it keeps coming back, no attack lands, and grindy sacrifice-and-recursion decks have leaned on that condition long after the era that produced it.