Agonizing Demise is a common Instant card from Duel Decks: Ajani vs. Nicol Bolas (DDH), collector number 66. Current market price: $0.09, TCGplayer market prices via Scryfall.
Kicker {1}{R} (You may pay an additional {1}{R} as you cast this spell.) Destroy target nonblack creature. It can't be regenerated. If this spell was kicked, Agonizing Demise deals damage equal to that creature's power to the creature's controller.