Summoner's Pact

A loan against your future, written in mana. Casting costs nothing now; you pull any green creature straight to hand and defer the entire bill to your next upkeep, where comes due or the game ends. The genius of the design is that it converts a tutor into a tempo instrument: the zero cost means you can search up a piece on a turn you have no spare mana, hold a card up that opponents must respect as a free instant, and only settle accounts after you have already cashed in. Combo decks understood this immediately. If the creature you fetch lets you win before that upkeep arrives, the is never paid because there is no next upkeep to pay it in: the clause is dead text by design, a price tag on a card you intend to break before it ever scans. That is the line the whole effect walks, between a real loan a fair deck must honor and an empty threat a fast deck simply outruns. The "you lose the game" downside is not really a downside in the hands that want it most; it is the cost the format trusts to keep the free tutor from being strictly free, and the cleanest possible demonstration of how deferred payment reshapes what an effect is worth.