Once you have all three Elder Scrolls (Sun, Blood, Dragon), the quest "Seeking Disclosure" updates. You need to bring the Scrolls to Dexion so that he can complete his reading. Return to Fort Dawnguard and talk to Dexion, the Moth Priest. You find out however, that there is a problem. Dexion has gone blind. He did not prepare enough for the reading of the first scroll, and this haste caused irreversible damage to his eyesight. He can no longer be of use in this matter. But there is another way. There are Ancestor Glades around Tamriel, including one in the Pine Forest in Skyrim. If you perform the Ritual of the Ancestor Moth inside that glade, you will get the answers you seek.

To perform this ritual, you first will need bark from a Canticle Tree. This will attract Ancestor Moths which will provide you with the second sight needed to understand the Scrolls. You will need to use a special knife called a Draw Knife to gather the bark.

This completes "Seeking Disclosure" and starts "Unseen Visions". Your first objective is to find a Moth's Priest knife. You will find this knife in the Ancestor Glade itself. Exit Fort Dawnguard and find the Ancestor Glade on your map. You probably have not discovered this location yet, so you cannot fast travel. The closest places you've probably been to is either Helgen (northeast of the Ancestor Glade) or Falkreath (SW of the Ancestor Glade). Fast travel to one of those locations, then set out for the Ancestor Glade on foot. Cross through the wilderness, and eventually a steep road up the mountain leads you to Ancestor Glade. Enter Ancestor Glade.

First, find the Draw Knife in the center of the Glade. Use the knife to gather bark from the Canticle Tree. After harvesting the bark, run around the glade and near the groups of moths until you attract seven of the Ancestor Moth Swarms. Then you will need to enter the column of light (near where you found the knife) and read the Elder Scroll (Blood). You will see a vision. Speak to Serana after you are done. You tell Serana that the vision has taught you the location of Auriel's Bow: in Darkfall Cave.

You are done with the quest "Unseen Visions",  and the next quest begins: Touching the Sky.