The prophecy is written between three separate Elder Scrolls. The inscription denotes a ritual which requires a Daughter of Coldharbour as a sacrifice and the bow of Auri-El to permanently purge the world of sunlight, allowing vampires to roam unhindered.
Arch-Curate Vyrthur, one of the last surviving Snow Elves, was afflicted with vampirism in the Merethic Era. He felt betrayed that Auri-El, the chief deity in his faith, would allow this to happen to him. Seeking revenge, he created a prophecy which, if enacted, would block out the sun for eternity. This would sever Auri-El's link between Mundus and Aetherius, thus weakening his influence.
During the Dawnguard questline, the Volkihar Clan begin terrorizing the people of Skyrim, impeded only by the sunlight. Lord Harkon, leader of the Volkihar, found traces of Vyrthur's prophecy in writings from a Moth Priest. Harkon sought the prophecy, but his efforts were cut short when his wife, Valerica, fled from him and took an Elder Scroll that contained part of the prophecy.
According to the Elder Scrolls, the items required are the Bow of Auri-El and the blood of a Daughter of Coldharbour. The bow was said to be used by Auri-El himself and had the power of the sun at its disposal. After collecting the bow, a blood-stained arrow must be shot into the sun, causing a mysterious energy to block out its rays. This energy, however, only lasts during daylight hours.