The Dragon-Kings are the seven god-like dragons, who rule over the city-states of the setting. Each dragon rules one city-state, and these states are loosely allied.
The Dragon-Kings came from another plane, which was devastated by a variety of cataclysmic events. In a search for a new home they travelled to this setting, and within a few years they had conquered the realms and sealed off all gateways to other planes, though out of the twenty dragons that arrived, by the end of the Kings’ War, only thirteen remained. They split the kingdom into seven city-states, which were loosely allied in order to protect them from those who would conspire against them. Over time the remaining dragons were killed off, until only eight remained, one for each of the city-states, and their progenitor.
Once the Dragon-Kings arrived on the plane, they found that their eggs no longer produced dragons, but instead, depending on the time of year, produced Drakes, Wyverns, and Dragonborn. As a result, as each dragon is killed, the species slowly dies out, at least on this plane.