When you set this scheme in motion, you may pay {X}. If you do, this scheme deals X damage to target opponent or planeswalker and each creature that player or that planeswalker's controller controls.
You choose the target opponent when the ability triggers. As the ability resolves, you choose a value for X and decide whether to pay {X}. If you do decide to pay {X}, it’s too late for any player to respond since the ability is already in the midst of resolving.