After performing the Moth Ritual in the Ancestor Glade and deciphering the Elder Scrolls as part of the quest "Unseen Visions", the next quest begins:

Touching the Sky
The vision provided by the Elder Scrolls seems to indicate that Auriel's bow may be located in a place known as Darkfall Cave. I should proceed there as soon as I'm prepared if I wish to recover the bow and stop Harkon from completing the Tyranny of the Sun.

Locate Auriel's Bow.

 You can ask Serana what she knows about Auriel's Bow. It has never been held by a vampire. If you ask her what the bow has to do with the suns, she tells you that Auriel is one of the Elven Gods. He is with the rest of them in Aetherios. The sun represents the connection from our world to theirs. The bow draws its energy from the sun itself, which is why it shows up in that prophecy.

As you prepare to leave the glade, you will be ambushed  by a gang of Dawnguards and Armored Frost Trolls. The Dawnguard has armored battle trolls that they use when fighting vampires such as you. These trolls are harder to kill than regular trolls.

Kill all the dawnguards in the glade. Then, exit the Ancestor Glade. Open your map and locate the Darkfall Cave.  There are several waypoints near it which you can use to fast travel, including Dragontooth Crater, Deepwood Redoubt, and Bruca's Leap Redoubt. Find the closest place you can fast travel to near Darkfall Cave. If you don't have any of those places, you can go to Solitude. After fast traveling, you should end up being pretty close to the Darkfall Caves. Travel by foot (or mount) and reach the Darkfall Cave.

Enter the Darkfall Cave.