According to Valerica, the scroll reveals that "The Blood of Coldharbor's Daughter will blind the eye of the Dragon". This refers to the Tyranny of the Sun, an ancient prophecy that foretells the eternal beclouding of the sun.
This scroll was originally in the possession of Valerica, Lord Harkon's estranged wife. She took it from Castle Volkihar some time before the events of Skyrim, and hid it, along with her daughter Serana in Dimhollow Crypt so that Harkon could not complete the prophecy.
"I see a vision before me, an image of a great bow. I know this weapon! It is Auriel's Bow! Now a voice whispers, saying "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 voice fades and the words begin to shimmer and distort. But wait, there is more here. The secret of the bow's power is written elsewhere. I think there is more to the prophecy, recorded in other scrolls. Yes, I see them now... One contains the ancient secrets of the dragons, and the other speaks of the potency of ancient blood.
My vision darkens, and I see no more. To know the complete prophecy, we must have the other two scrolls."