Oketra's Monument is a uncommon Legendary Artifact card from Amonkhet (AKH), collector number 233. Current market price: $3.33 (foil $16.00), TCGplayer market prices via Scryfall.
White creature spells you cast cost {1} less to cast. Whenever you cast a creature spell, create a 1/1 white Warrior creature token with vigilance.