Cavern
Cita:
As Cavern of Souls enters the battlefield, choose a creature type.

: 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.
Counterbalance
Cita:
Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, counter that spell if it has the same converted mana cost as the revealed card.
Entra in gioco la "regola d'oro" di MTG.
101. The Magic Golden Rules
101.1. Whenever a card's text directly contradicts these rules, the card takes precedence. The card overrides only the rule that applies to that specific situation. The only exception is that a player can concede the game at any time (see rule 104.3a).
101.2.
When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence.