War Cadence

• WWizards of the Coast2004-10-04
  Official Ruling
  The ability only applies to blocks declared after it resolves. It will not add costs to any blockers already announced.
• WWizards of the Coast2004-10-04
  Official Ruling
  You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t block unless the controller chooses to pay the cost (a cost of zero is not automatically paid). You can use this to make your creatures unable to block an attacker.

Rulings via Scryfall