Polymorph

The card that gave a whole archetype its name. The text reads as a removal spell with a downside (you kill a creature, but the controller gets a replacement straight off the top of their library), and that downside is exactly the engine builders learned to exploit. Aim it at your own token and the dig clause becomes a tutor: build a deck whose only creature is one bomb, drop a 0/1 chump token to point it at, and Polymorph cheats a haymaker into play for four mana years before the board even has a fair body to find. The load-bearing line is "until they reveal a creature card": the reveal stops at the first creature found, so a singleton creature count turns a random dig into a deterministic one (run zero other creatures and the only thing the reveal can hit is the bomb). That constraint is what makes the rate work as a combo piece rather than a card-disadvantage trap, and it is why the blocker has to be a token or manland; any second creature in the library breaks the math. The deckbuilding cost is real, but it buys determinism. Later effects in the same vein (Indomitable Creativity sweeping multiple targets, Proteus Staff stacking the dig) all answer the same question this template posed first: a removal spell aimed at your own permanent can be the most efficient cheat-into-play effect in the game, provided you build the library to deserve it.