Dragon lords of Jund rule by edicts of flame.

Flameblast Dragon

Flying
Whenever this creature attacks, you may pay {X}{R}. 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.

Starter Commander Decks

Game Night: Free-for-All

Flying
Whenever Flameblast Dragon attacks, you may pay {X}{R}. If you do, Flameblast Dragon deals X damage to any target.

Commander 2018

Magic 2012

Flying
Whenever Flameblast Dragon attacks, you may pay {X}{R}. If you do, Flameblast Dragon deals X damage to target creature or player.

Salvat 2011

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.