Summoner's Pact is a rare Instant card from Mystery Booster 2 (MB2), collector number 74. Current market price: $3.19, 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.