August 25, 2015 Wizards of the Coast
Effects that increase or reduce the cost to cast a spell don't affect that spell's mana value.
August 25, 2015 Wizards of the Coast
The mana value of a face-down creature spell or a face-down creature is 0. Void Winnower will stop face-down creature spells from being cast and face-down creatures from blocking.
August 25, 2015 Wizards of the Coast
Yes, your opponent can't even. We know.
August 25, 2015 Wizards of the Coast
For creatures on the battlefield with in their mana costs, the value for X is 0. Mistcutter Hydra (a creature with mana cost ) could block, but Endless One could not.
August 25, 2015 Wizards of the Coast
For spells with in their mana costs, use the value chosen for X to determine if the spell's mana value is even or not. For example, your opponent could cast Endless One (with mana cost ) with X equal to 5, but not with X equal to 6.

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