After finding the entrance to the Ancestor Glade and going in, proceed through some caverns until emerging in a beautiful open area with trees and a waterfall. Locate a stone pedestal holding the following item:

Draw Knife

Once you have picked up the knife from the stand, your quest objective updates. You need to gather some bark from a Canticle tree. Harvest Canticle Bark from one of the Canticle trees nearby.

Next, your objective is:
Attract Ancestor Moth Swarms (0/7)

How to attract Ancestor Moth Swarms
Run around the glade, and when you see a Moth Swarm, run toward it. Once you get close to a Moth Swarm, you attract it and advance the objectives. Keep running around the Ancestor Glade until you have attracted all seven Moth Swarms.

Once you have attracted the 7 Ancestor Moth Swarms, your next objective is to enter the column of light and read the Elder Scroll (Blood).

When you are inside the light column, open your inventory and select the Blood Elder Scroll (note that Elder Scrolls are found under "Books" rather than "Scrolls"). Experience the vision. Then, speak to Serana. She asks you if you know where Auriel's Bow is. You tell her it's in a place called Darkfall Cave. Your map should now be updated with the location of the cave.

This completes the quest "Unseen Visions", and starts the quest "Touching the Sky"