Your objective, as part of the quest "Touching the Sky" is to locate Auriel's Bow, which you now know lays inside Darkfall Cave. Head towards the exit. There is an ambush waiting for you before you leave the Glade including Vampire Thralls, Volkihar Vampires, Master Vampires, and Gargoyle Brutes. After you clear the enemies, exit Ancestor Glade. Open the map, and find the point closest to Darkfall Cave to which you can fast travel, possibly Dragontooth Crater or one of the many Redoubts in the area.

Enter Darkfall Cave. The caverns are narrow, and you will be fighting spiders throughout. When you reach a wooden bridge, jump down into the water below. Dive to go deeper into the tunnel. If you are in Vampire Lord Form, you will have to revert at this point, so that you can swim. Watch out for avalanches as you advance through the corridors. Soon you reach a big indoors chantry. This is the Great Chantry of Auri-El. Defeat the Trolls scattered around it, and speak to Knight-Paladin Gelebor, a snow elf.

Gelebor tells you that he will help you get Auriel's Bow, if you help him kill his brother, Arch-Curate Vyrthur. He shows you a Wayshrine, one of many shrines used for a ritual required to gain access to the Inner Sanctum, where Vyrthur and the Bow are. Initiates would fill their ewer from each of the Wayshrines, and the pour the water into the sacred basin of the Sanctum. This is what you will have to do to be able to enter the Inner Sanctum.

Gelebor says that the first Wayshrine is at the end of Darkfall Passage. There are 5 shrines in total. He gives you the initiate's ewer. Go inside the shrine near Gelebor and enter the Darkfall Passage Portal. Fight your way through the passages, there will be many Falmer enemies in this zone. Arrive at a big chamber with a chest under a bridge, but watch out since this is a trap. Go up and over the bridge, and exit the chamber through a gate to the west. Go south, then east through a corridor and into a cavern with a big waterfall in it. Cross the bridge on the left to the other side of the waterfall. Take another narrow tunnel to the east. Activate a rope release on the wall to reveal the next part of the tunnel.

There will be another big cavern with jellyfish like plants. Go through the small body of water in the center of this chamber, then take a bridge south. Make your way up some platforms that soon turn west, and you arrive at the Wayshrine of Illumination. Talk to the Spectral Prelate there, and fill the initiate's ewer from the basin inside the shrine.

Take the portal to the Forgotten Vale.