Cavern of Souls | Magic: The Gathering
Formats Cavern of Souls is Legal in
Official Oracle Text for Cavern of Souls
: Add to your mana pool.
: 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.
Official Rulings for Cavern of Souls
5/1/2012 : 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."