During the Oblivion Crisis at the end of the Third Era, the Mythic Dawn was a cult created on behalf of the daedric prince Mehrunes Dagon in order to sow chaos in the name of change, eventually to help him to enter the mortal realm in his true form to challenge the Gods for the control of Tamriel.
The Mythic Dawn became famous when they murdered not only Uriel Septim VII, but also all of his known heirs. They were led by Mankar Camoran, who was under the belief that Tamriel was actually a plane of Oblivion that the Aedra stole from Lorkhan after the creation of Mundus. He stole the Amulet of Kings and escaped into his Paradise, only to be chased by the Hero of Kvatch. He was slain in Paradise, and the Amulet was returned to the last Septim heir — Martin Septim.
After the Oblivion Crisis ended, the Mythic Dawn was completely wiped out, and there was a group that emerged to counteract the idea of a faction such as the Mythc Dawn ever rising again. The Vigilants of Stendarr went across all of Tamriel in attempt to wipe out any Daedric influence. There was one known descendant of the Mythic Dawn in the Third Century of the Fourth Era, who opened a museum dedicated to the Mythic Dawn.