Lotus Petal

Black Lotus split into thirds and reassembled as a one-shot: the design trades two-thirds of the original's burst for the same defining trait, which is that it costs nothing to cast. That last part is why combo decks willing to pay a card for tempo keep reaching for it. It accelerates from zero to one mana of any color the instant you need it, fixes a color you are short on, and crucially adds to your storm count or artifact count for free before it disappears. The sacrifice clause is what keeps the card from becoming a ramp engine: it is a single payment you cash in at the exact moment a combo needs to fire, which is why it lives in glass-cannon shells rather than fair midrange. The zero cost also means it never sets you back on the tempo curve to deploy it; you can crack it the turn it enters or hold it as a deferred mana source that doubles as fuel for anything counting permanents leaving the battlefield. It belongs to the small family of mana rocks that ask nothing up front and everything at once, alongside Lion's Eye Diamond and the moxen, where the question is never whether the mana is efficient but whether you can convert it into a win before the artifact is gone.