Killer Instinct

The bargain is one-sided in a precise way: each upkeep you flip the top of your library, and if it is a creature, it crashes into play for free with haste, only to be sacrificed at the beginning of the next end step. Miss, and the revealed noncreature card stays on top exactly until your draw step a moment later, where you simply draw it: the whiff costs you nothing but a turn without a body, and it doesn't clog future flips. The design lives entirely in the gap between the size of the swing and your total lack of control over it. You cannot choose which creature you get, and you cannot keep it, so this rewards bodies whose value is front-loaded rather than persistent: an attack trigger, an enters-the-battlefield effect, a frame big enough to trade up in a single swing. Because the creature lingers through the post-combat main phase before dying at end step, the forced sacrifice is softer than it looks: a death-trigger payoff or a sacrifice outlet converts the loss into a payout, since you were never keeping the thing anyway. It cheats threats off the top of your deck instead of out of your hand, paid for in randomness rather than mana, and the whole engine asks whether you can stomach letting your library pick your attacker for you, knowing some turns it picks nothing at all.