Knight of Obligation is a uncommon Creature — Human Knight card from Gatecrash (GTC), collector number 18. Current market price: $0.19 (foil $2.84), TCGplayer market prices via Scryfall.
Vigilance Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)