Cavern of Souls

Oracle Text

As Cavern of Souls enters the battlefield, choose a creature type.

T: Add C to your mana pool.

T: Add one mana of any color to your mana pool. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.

Card Rulings

3/14/2017 You must choose an existing creature type, such as Zombie or Warrior. Card types such as artifact can’t be chosen.
3/14/2017 The spell can’t be countered if the mana produced by Cavern of Souls is spent to cover any cost of the spell, even an additional cost such as a kicker cost. This is true even if you use the mana to pay an additional cost while casting a spell “without paying its mana cost.”