Void Winnower

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Effects that increase or reduce the cost to cast a spell don't affect that spell's mana value.

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.

Yes, your opponent can't even. We know.

For creatures on the battlefield with {X} in their mana costs, the value for X is 0. Mistcutter Hydra (a creature with mana cost {X}) could block, but Endless One could not.

For spells with {X} 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 {X}) with X equal to 5, but not with X equal to 6.