Putrid Leech

A 2/2 for two that becomes a 4/4 the moment you decide to bleed for it, which makes it one of the cleanest articulations of the life-as-resource gambit in two colors. The body it ships with is unremarkable; the card it becomes when you pay 2 life is a 4/4 for two mana, a rate that aggressive black-green decks have always been willing to trade life for. The once-per-turn clause is the leash: it caps the pump at a single activation, so the Leech threatens exactly four damage rather than scaling out of control, and that ceiling is what lets the toll stay as cheap as 2 life instead of demanding more. The pump resolves at instant speed, which is the wrinkle that gives it teeth in combat. You hold the activation until blocks are declared, then decide whether 2 of your life is worth trading for a better attack or a surviving blocker, turning every swing into a question the defender has to answer before the answer is knowable. The arithmetic is brutally honest: you are spending your own life total to close games faster, and against a deck racing back at you that same 2 life can flip from cheap acceleration to a real liability in a single turn. The design asks nothing of your deck beyond a willingness to live dangerously.