As Sanctum Prelate enters the battlefield, choose a number.
Noncreature spells with converted mana cost equal to the chosen number can't be cast.
8/23/2016 Effects that increase or reduce the cost to cast a spell don’t affect that spell’s converted mana cost.
8/23/2016 For spells with in their mana costs, use the value chosen for X to determine if the spell’s converted mana cost is the chosen number. For example, if the chosen number is 4, a noncreature spell with mana cost couldn’t be cast with X equal to 2, but it could be cast with X equal to any other number.