Dragon Cult

The Dragon Cult  was an order of Dragon worshippers in Merethic and early First Era. The highest ranking worshipers were Dragon Priests, which were granted artifacts of immense power by the Dragons themselves. Their power equaled to the power of kings.

History
The Dragon Priests on Atmora set laws and codes that kept peace between Men and Dragons. Their kingdom was prosperous. When they migrated to Tamriel, they became tyrants, making slaves of free people and torturing them. It is unknown why this happened, and it is theorized that it because of specific Dragon Priest or Dragon that they became that way.

Dragon War
When the Dragon War erupted, the dragon priests retaliated. They couldn't control the masses and thus asked the dragons for help. Dragons' response was swift and brutal. Men couldn't do anything against their Voice. Ancient texts suggest few dragons sided with Men. No one is certain why. Priests of the Nine Divines believe Akatosh himself intervened. Kynareth, on the other hand, pitied the humans and sent Paarthurnax, who also pitied them, to teach Men the Voice. The tide began to turn and more and more dragons were killed. In the end, the dragons and the Dragon cult were overthrown. Dragons that managed to escape, hid themselves away from Men. The Dragon cult managed to survive for some time longer. The remaining members buried dragons in hope of their rising again.

City of Bromjunnar (Labyrinthian)
The city of Bromjunnar was the place where highest ranking Dragon Priests gathered and discussed matters of ruling. It was originally built as temple for Dragons, but after the Dragon War, it fell into ruin. In the First Era, Arch-Mage Shalidor used part of it to serve as a testing ground for future Arch-Mages of the College of Winterhold. It is believed that it was the capital of Skyrim during the Dragon Cult's highest period of influence.

Known Dragon priests
List of the most powerful Dragon Priests that lived in Skyrim and the location in which they were buried:

Hevnoraak - Valthume

Morokei - Labyrinthian

Nahkriin - Skuldafn

Krosis - Shearpoint (mountain)

Rahgot - Forelhost

Otar the Mad - Ragnvald

Vokun - High Gate Ruins

Volsung - Volskygge

On Solstheim:

Miraak - Apocrypha, he didn't die, but was transported to Hermaus-Mora's realm of Oblivion

Vahlok the Jailer - Vahlok's Tomb

Dukaan - White Ridge Barrow

Zahkriisos - Bloodskal Barrow

Ahzidal - Kolbjorn Barrow