Fungal Infection

The same mana that buys a one-turn shrink usually buys nothing else, but here it leaves a Saproling behind, and that conversion is the entire design. The spell trades the temporary downside of a cheap removal effect (it kills X/1s, shrinks a blocker, or finishes something already wounded) for permanent board presence, so the card-disadvantage that normally taxes small combat tricks and marginal removal simply does not land. You spend a card and a mana, you end the exchange with a creature. The -1/-1 until end of turn is doing more than dealing damage: it is a toughness-reduction effect that can zero out a creature's power or kill something clinging to its last point of toughness, and it happens at instant speed, so it doubles as a combat blowout or an answer on the opponent's turn. The token is what makes all that flexibility cheap to justify; even when the shrink is a throwaway (clearing a chump, ticking down a token), you still come out ahead on bodies. The principle underneath is quiet but exact: removal that replaces itself stops functioning as a real resource cost, and a one-mana instant that leaves a creature on the table earns its keep in any deck built around sacrifice fodder, going wide, or grinding out incremental value one body at a time.