Grimoire Thief is a rare Creature — Merfolk Rogue card from Morningtide (MOR), collector number 35. Current market price: $0.68 (foil $4.92), TCGplayer market prices via Scryfall.
Whenever this creature becomes tapped, exile the top three cards of target opponent's library face down. You may look at cards exiled with this creature. {U}, Sacrifice this creature: Turn all cards exiled with this creature face up. Counter all spells with those names.