I know this looks very similar to this question, but I asked it to indirectly solve what I'm about to ask now (a bad idea).
The Aegis of the Raven Queen (from the module "CCC-BWM-03 A Tale of Two Towers") is a special shield, which does not exist in D&D 5e by default but does exist in Adventurers League play. It has the following passive property:
While holding this shield, nonmagical flames are extinguished within 30 feet of you as the shadow lashes out at the flame.
I also know that breath weapons are considered nonmagical.
What happens when a dragon breathes fire on a creature wielding this special shield?
- The breath gets extinguished (and thus doesn't affect the Aegis-wielding creature)
- The breath weapon works as normal, since it doesn't really emit "flames" in game terms (and thus the Aegis-wielding creature is forced to make the Dexterity saving throw as normal)