Void Winnower — ruling 1

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

Void Winnower — ruling 2

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.

Void Winnower — ruling 3

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.

Void Winnower — ruling 4

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.

Void Winnower — ruling 5

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

Void Winnower : règles, fonctionnement et interactions (MTG)

Void Winnower is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Your opponents can't cast spells with even mana values. (Zero is even.) Your opponents can't block with creatures with even mana values.

How it works : Void Winnower

Original English text — Wizards of the Coast / Scryfall

#1August 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.
No other card is named in this ruling.
#2August 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.
No other card is named in this ruling.
#3August 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.
No other card is named in this ruling.
#4August 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.
No other card is named in this ruling.
#5August 25, 2015· Wizards of the Coast
Yes, your opponent can't even. We know.
No other card is named in this ruling.