Cavern of Souls is a rare Land card from The List (PLST), collector number AVR-226. Current market price: $52.60, 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.