Cavern of Souls

Text:

As Cavern of Souls enters the battlefield, choose a creature type.
: Add {C}.
: 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.

Rulings:

• 12/7/2018 You must choose an existing creature type, such as Human or Warrior. Card types such as artifact can’t be chosen.
• 12/7/2018 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.”