Cavern of Souls is a mythic Land card from Modern Masters 2017 (MM3), collector number 232. Current market price: $50.62 (foil $59.51), TCGplayer market prices via Scryfall.
As this land enters, choose a creature type. {T}: Add {C}. {T}: Add one mana of any color. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.