Riddles in the Dark is a rare Instant card from Spotlight Series (PSPL), collector number 12. Current market price: $55.11 (foil $501.89), TCGplayer market prices via Scryfall.
Look at the top four cards of your library and separate them into a face-down pile and a face-up pile. An opponent chooses one of the piles. Put that pile into your hand and the other into your graveyard.