The Tyranny of the Sun was an ancient prophecy written in the First Era by Arch-Curate Vyrthur of the Chantry of Auri-El. The prophecy foretold of a time when the sun would be blotted from the sky and the world plunged into eternal darkness. The prophecy was recorded across three separate Elder Scrolls and its fulfillment required Auriel's Bow and the sacrifice of a Daughter of Coldharbour.
A Snow Elf named Vyrthur served as the Arch-Curate at the Chantry of Auri-El during the First Era. His position in the Chantry was such that he was said to commune with Auri-El himself. When he was afflicted with vampirism by one of the initiates of the Chantry, he felt betrayed that Auri-El would allow such a thing to happen to him.
Vyrthur sought revenge on Auri-El by creating a prophecy which, if fulfilled, would becloud the sun for eternity. Such an act would sunder the connection between Aetherius, the plane of the gods, and Mundus. This in turn would eliminate Auri-El's influence on Nirn. This was to be achieved by corrupting Auriel's Bow and using it to loose an arrow, soaked in the blood of a Daughter of Coldharbour, into the sun. Vyrthur began to publicize the prophecy with the intent of drawing a Daughter of Coldharbour to him.
Years later, the prophecy was revealed to a Moth Priest upon reading an Elder Scroll. The Moth Priest began to transcribe his findings but died before it could be completed. He did however identify the three Elder Scrolls that contained the prophecy.
Harkon, an ancient pure-blood vampire, became aware of the prophecy when he uncovered the Moth Priest's writings. As a vampire, the sun was his greatest enemy and for many years he had sought a solution to this problem. When he discovered that the fulfilled prophecy would plunge the world into eternal darkness, he became obsessed with its realization. He came to possess two of the Elder Scrolls the Moth Priest had identified, one of which declared that "The Blood of Coldharbour's Daughter will blind the eye of the Dragon." Realizing that his own daughter, Serana, could be used for the blood sacrifice, he sought to learn more. Before Harkon could proceed any further, his wife fled from him and took Serana and the Elder Scrolls with her. She hid one scroll with Serana deep within an ancient Nordic tomb and escaped to the Soul Cairn with the other. As a result, the prophecy lay dormant for the next several thousand years.
In 4E 201, Harkon discovered the whereabouts of Serana and the Elder Scroll and sent a group of his followers to retrieve her. Instead, she was released from her tomb and escorted back to her father by the Last Dragonborn. With Serana's Elder Scroll returned to him, Harkon kidnapped a Moth Priest in order to decipher the prophecy. The Moth Priest eventually revealed another part of the prophecy that was congruous with the events of the time: "Among the night's children, a dread lord will rise. In an age of strife, when dragons return to the realm of men, darkness will mingle with light and the night and day will be as one." The priest also revealed that the fabled bow of Auri-El was somehow involved, but without the other two scrolls its location remained a mystery.
After retrieving the other two Elder Scrolls, one from the Soul Cairn and the other from deep within a Dwemer ruin, the Last Dragonborn used all three to perform the Ritual of the Ancestor Moth within a secluded ancestor glade. The ritual revealed the location of Auriel's Bow, and the Dragonborn subsequently traveled with Serana to retrieve it. The bow was located in the Chantry of Auri-El, guarded by Arch-Curate Vyrthur himself. By luring Serana directly to him, Vyrthur's vengeance against Auri-El almost came to fruition; however, Serana and the Last Dragonborn were able to defeat and kill Vyrthur. With the bow in the possession of the Dragonborn, it is unknown if the prophecy was stopped or reshaped.