Abstract Performance (Secrets of Strixhaven Commander)

Sorcery - Exile the top four cards of your library in a face-down pile, then exile the top four cards of your library in a face-up pile. An opponent chooses one of those piles. Put that pile into your graveyard. Look at the cards in the other pile. You may cast a spell from among them without paying its mana cost. Put the rest into your hand.