Kefnet's Monument is a uncommon Legendary Artifact card from Amonkhet (AKH), collector number 231. Current market price: $0.38 (foil $2.66), TCGplayer market prices via Scryfall.
Blue creature spells you cast cost {1} less to cast. Whenever you cast a creature spell, target creature an opponent controls doesn't untap during its controller's next untap step.