Void Winnower

• WWizards of the Coast2015-08-25
  Official Ruling
  Effects that increase or reduce the cost to cast a spell don't affect that spell's mana value.
• WWizards of the Coast2015-08-25
  Official Ruling
  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.
• WWizards of the Coast2015-08-25
  Official Ruling
  Yes, your opponent can't even. We know.
• WWizards of the Coast2015-08-25
  Official Ruling
  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.
• WWizards of the Coast2015-08-25
  Official Ruling
  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.

Rulings via Scryfall