2018-12-07 : 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."

2018-12-07 : You must choose an existing creature type, such as Human or Warrior. Card types such as artifact can't be chosen.