User blog comment:Jimeee/Dragonborn officially released on Xbox Live/@comment-5179958-20121213195226/@comment-3202167-20121217141944

Here ya go: The Dragon Priests themselves can be traced back to Atmora where they were the ruling body during the Merathic Era. Known as theDragon Cult, they worshipped many animals as avatars of the gods, the Dragon being chief among them.

Dragons embraced their role as god-kings over men, but rather than deal with the actual ruling itself, they granted small amounts of power to dragon priests in exchange for absolute obedience. In turn, the priests ruled men as equals to the kings. The priests set down laws and codes of living that kept peace between Dragons and men, but to do so they demanded tribute.

When the Atmorans began to migrate to Tamriel over the next several centuries, the dragon priests and cultists were some of the first men to arrive. The system of rule in Tamriel however was quite different from Atmora. It is unclear why this was the case, but the priests began to rule with an iron fist, making slaves of the rest of the population.

From the Wiki.