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Your opponents can't cast spells with even mana values. (Zero is even.)
Your opponents can't block with creatures with even mana values.
Effects that increase or reduce the cost to cast a spell don’t affect that spell’s converted mana cost.
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}{G}) 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 converted mana cost 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.
The converted mana cost 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.
