Thief of Sanity is a rare Creature — Specter card from Ravnica: Clue Edition (CLU), collector number 212. Current market price: $0.38, TCGplayer market prices via Scryfall.
Flying Whenever this creature deals combat damage to a player, look at the top three cards of that player's library, exile one of them face down, then put the rest into their graveyard. You may cast that card for as long as it remains exiled, and mana of any type can be spent to cast that spell.