Forbidden Alchemy is a uncommon Instant card from Ultimate Masters (UMA), collector number 56. Current market price: $0.26 (foil $0.10), TCGplayer market prices via Scryfall.
Look at the top four cards of your library. Put one of them into your hand and the rest into your graveyard. Flashback {6}{B} (You may cast this card from your graveyard for its flashback cost. Then exile it.)