Rhonas's Monument is a uncommon Legendary Artifact card from Amonkhet (AKH), collector number 236. Current market price: $2.12 (foil $6.28), TCGplayer market prices via Scryfall.
Green creature spells you cast cost {1} less to cast. Whenever you cast a creature spell, target creature you control gets +2/+2 and gains trample until end of turn.