Cavern of Souls
As Cavern of Souls enters the battlefield, choose a creature type. : Add . : 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.
237 UMA • ENRichard Wright
™ & © 1993— Wizards of the Coast
Notes: TODO: Update Copyright
Legal in: Modern,Innistrad Block,Legacy,Vintage,Freeform,Prismatic,Tribal Wars Legacy,Tribal Wars Standard,Singleton 100,Commander
Oracle Text (click to copy):
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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."