Tablet of Discovery is a uncommon Artifact card from Secrets of Strixhaven (SOS), collector number 132. Current market price: $1.08 (foil $2.26), TCGplayer market prices via Scryfall.
When this artifact enters, mill a card. You may play that card this turn. (To mill a card, put the top card of your library into your graveyard.) {T}: Add {R}. {T}: Add {R}{R}. Spend this mana only to cast instant and sorcery spells.