Summoner's Pact is a rare Instant card from Masters 25 (A25), collector number 189. Current market price: $5.22 (foil $23.94), TCGplayer market prices via Scryfall.
Search your library for a green creature card, reveal it, put it into your hand, then shuffle. At the beginning of your next upkeep, pay {2}{G}{G}. If you don't, you lose the game.