I think a breath weapon is just that... what it uses to combat evil.
Dragons typically have breath weapons. The most common of course is breathing fire. Dragons from the Dungeons and Dragons game, for instance, have different breath weapons based on their color. Some spit acid or emit a bolt of lightning or spew a cloud of poisonous gas.
But for these Beatific Dragons, they should have a breath weapon that goes along with their saintly, holy, or benevolent natures.
At the same time, these are dragons... they just happen to fight for goodness and not evil.