Desperate Gambit is a uncommon Instant card from Weatherlight (WTH), collector number 96. Current market price: $0.60, TCGplayer market prices via Scryfall.
Choose a source you control and flip a coin. If you win the flip, the next time that source would deal damage this turn, it deals double that damage instead. If you lose the flip, the next time it would deal damage this turn, prevent that damage.