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.Cavern of Souls helps a deck built around a creature type cast its important creatures through countermagic. You choose a type as the land enters, then its colored-mana ability can help cast a creature of that type while making that spell unable to be countered.
Dragon decks illustrate the benefit well. Naming Dragon can protect a cast of Miirym, Sentinel Wyrm or Lathliss, Dragon Queen, helping establish the creature that makes later Dragons more productive. In a Vampire deck, naming Vampire can support Edgar Markov and the creatures surrounding him. The land’s flexibility comes from choosing the type that matches the deck you are building.
You must actually spend mana from Cavern’s restricted colored-mana ability on the creature spell to receive the protection from countering. Merely controlling Cavern is not enough. Its separate colorless-mana ability can help pay for other spells or abilities, but does not grant that benefit. The colored mana cannot be used for an unrelated noncreature spell just because that spell supports your chosen creature type.
Cavern does not protect a creature from removal after it resolves, and opponents can still interact with its triggered abilities. Lightning Greaves or Swiftfoot Boots can provide ongoing protection for a valuable commander once it is on the battlefield. Keep enough ordinary colored sources for removal, ramp, and other noncreature spells as well. Cavern is strongest when a large share of your important creatures shares a type, making both the color fixing and casting protection relevant throughout the game.
