They engaged in a war against dragons, for about a thousand years of their history. All over a betrayal by those mortals the dragons had come to trust, no less. The techniques and battle styles of the dragoon were developed while fighting upon the backs of dragons, yes, but they eventually were turned against them in their crusade to conceal the truth.

[Irhya looks off into the distance, thoughtful.]

Dragons in my world are effectively immortal. So the conspirators killed one of members of the First Brood, hoping to partake of her eternal life. Her murder shattered the peace between them and incited the rage of her brother, who vowed a never-ending vengeance upon them for many generations to come.

And to think, the eternal life thing didn't even work out in the first place. It just made everything worse for their descendants.