The Dragonplague

The Dragonplague was a major epidemic that effected only dragons and other reptilians, especially dragonborn, in and around the Aurean Empire between 1734 and 1737 AY. It devestated the ruling dragon population of the empire, likely leading to the end of the Age of the Dragon Emperors, and paved the way for eventual rule by humans.
Plagues among dragons have historically been rare, as they tend to be solitary. However, the Aurean Empire at this time was ruled by dragons, and it spread quickly among them. The plagues symptoms came on rapidly, often killing the infected within days of the symptoms appearing. As a result the dragons largely put more distance between themselves and most deaths happened within the first year — though deaths continued after, with the emperess Eraxima succumbing in 1736. A majority of dragons affiliated with the empire did survive, but many abandoned the empire for their own safety, often never returning. Dragon rule would limp on for some time but never fully recovered.
The Dragonborn population was devestated as well. They were affected as strongly as the dragons, if not more so, and were left with a smaller and more spread out population than before. While some cities, such as Terminus, were said to be majority dragonborn before the plague, afterwards their population became incredibly thin and disperate. The dragonborn population was already much smaller than the human one, but was reduced to large enough degree that the dragonborn, who were generally of higher status in the empire, were not a factor in human control of Aurea when the dragons ultimately left.