Forbidden Alchemy is a common Instant card from Innistrad (ISD), collector number 55. Current market price: $0.08 (foil $0.51), 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.)