Betrayer's Bargain is a uncommon Instant card from Duskmourn: House of Horror (DSK), collector number 126. Current market price: $0.15 (foil $0.16), TCGplayer market prices via Scryfall.
As an additional cost to cast this spell, sacrifice a creature or enchantment or pay {2}. Betrayer's Bargain deals 5 damage to target creature. If that creature would die this turn, exile it instead.