2015-08-25 : Effects that increase or reduce the cost to cast a spell don’t affect that spell’s converted mana cost.

2015-08-25 : 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.

2015-08-25 : 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.

2015-08-25 : 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.

2015-08-25 : Yes, your opponent can’t even. We know.