Cavern of Souls is a mythic Land card from Ultimate Box Topper (PUMA), collector number U32. Current market price: — (foil $148.04), 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.