Demonfire is a rare Sorcery card from RNA Guild Kit (GK2), collector number 56. Current market price: $0.32, TCGplayer market prices via Scryfall.
Demonfire deals X damage to any target. If a creature dealt damage this way would die this turn, exile it instead. Hellbent — If you have no cards in hand, this spell can't be countered and the damage can't be prevented.