Cavern of Souls
Land
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.
Illustrated by
Ultimate Masters#237MythicEnglishLegality
Rulings
Dec 7, 2018
You must choose an existing creature type, such as Human or Warrior. Card types such as artifact can’t be chosen.
Dec 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.”