Flameblast Dragon
Creature — Dragon
Flying
Whenever this creature attacks, you may pay 
. If you do, it deals X damage to any target.
He marks his territory with fire and wrath, etching the mountainside with his charring breath.
Jaime Jones
Magic 2012 (M12)
• #133Rulings
You choose the target when the ability triggers. When the ability resolves, you choose a value for X and decide whether to pay {X}{R}. If you do decide to pay {X}{R}, it's too late for any player to respond since the ability is already in the midst of resolving.
(wotc • 2008-10-01)
Rulings
You choose the target when the ability triggers. When the ability resolves, you choose a value for X and decide whether to pay {X}{R}. If you do decide to pay {X}{R}, it's too late for any player to respond since the ability is already in the midst of resolving.
(wotc • 2008-10-01)