Cavern of Souls is a rare Land card from Avacyn Restored (AVR), collector number 226. Current market price: $48.09 (foil $91.18), 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.