Sanguine Spy

The delayed draw trigger is the whole design, and its condition is the tell: not how many cards sit in your graveyard, but how many different mana values they cover. That is a diversity check, not a size check, and it steers you away from a bin stuffed with identical cheap spells toward a spread of costs. The sacrifice line feeds it, pitching another creature to surveil while helping you dig, so the card prefers to eat a mix of bodies rather than the same one-drop over and over. That five-value gate does real balancing work: it keeps the engine dormant in the early game, exactly when a free card would matter most, and only switches on once you have committed cards of varied cost to the yard. Everything else is combat texture. Menace and lifelink turn a modest 2/3 into a body that pressures blockers and offsets the two life the draw costs, so the attrition shell that fills a graveyard is also the shell that can afford to pay for cards. Note the limit, though: the activated ability demands another creature to sacrifice, so this is a payoff for a sacrifice deck, not the engine that supplies one. It converts fodder into selection and pays you off at end step, but the fodder has to come from somewhere else.