Geistflame Reservoir is a rare Artifact card from Innistrad: Midnight Hunt (MID), collector number 355. Current market price: $0.33 (foil $0.35), TCGplayer market prices via Scryfall.
Whenever you cast an instant or sorcery spell, put a charge counter on this artifact. {1}{R}, {T}, Remove any number of charge counters from this artifact: It deals that much damage to any target. {1}{R}, {T}: Exile the top card of your library. You may play that card this turn.