Land As Cavern of Souls enters the battlefield, choose a creature type.
Rare
{T}: Add {1} 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.
Illus. Cliff Childs

5/1/2012: You must choose an existing _Magic_ creature type, such as Zombie or Warrior. Card types such as artifact can't be chosen.
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."
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Extended (Type 1.X)
Legal in Standard (Type 2)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern