Requiting Hex is a uncommon Instant card from Lorwyn Eclipsed (ECL), collector number 116. Current market price: $0.66 (foil $1.26), TCGplayer market prices via Scryfall.
As an additional cost to cast this spell, you may blight 1. (You may put a -1/-1 counter on a creature you control.) Destroy target creature with mana value 2 or less. If this spell's additional cost was paid, you gain 2 life.