Forbidden Alchemy is a common Instant card from Tarkir: Dragonstorm Commander (TDC), collector number 152. Current market price: $0.25, 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.)