Seven Kingdoms: Seowyn's Crossing
The Dragon Emperor was ther name given to the ruler of the empire of Azmartheon. Although the rulers were not actual Dragons, they were Dragonborn, and rumors sometimes said that they were advised by dragons. With the fall of Azmartheon, the title has fallen largely into obscurity. Some people use the term a bit mockingly, such as “And then you’ll be named Dragon Emperor” to suggest that something is unlikely to happen.