After killing Vyrhur, Knight-Paladin Gelebor agrees to finally give you Auriel's Bow. He tells you that if you wish to learn more about the bow, or obtain Sunhallowed Arrows for it, he is more than happy to help.

Your final objective in the quest "Touching the Sky" is to retrieve Auriel's Bow. The bow is inside the pagoda near where Gelebor is standing, at the base of the balcony.

Skyrim Dawnguard Auriel's Bow

Once you have retrieved it, you will see:
Sunhallowed Elven Arrow (12) Added
Elven Arrow (12) Added
Auriel's Bow Added

You receive the following items:

Auriel's Bow (Legendary)
Damage: 86/Weight: 11/Value: 1436
20 points of Sun Damage. Undead targets take triple damage

Auriel's Bow Skyrim Dawnguard

Sunhallowed Elven Arrow
Causes sunburst attacks to nearby targets if shot at the sun with Auriel's Bow.

Skyrim Dawnguard Sunhallowed Arrows

Portals to all wayshrines in the Forgotten Vale will now be unlocked, and you can travel to any of the shrines now if you wish to explore more.

This completes the quest, "Touching the Sky", and starts the final vampire lord quest in the Dawnguard DLC:
Kindred Judgment

After taking the bow, talk to Gelebor again and you should see the following option:
"What can you tell me about Auriel's Bow?"

Choose this option to learn more about the bow. The bow was said to be carried by Auri-El himself into battle against the forces of Lokhan in ancient and mythic times. Its craftsmanship has no equal anywhere within Tamriel and possibly beyond. The bow draws its power from Aetherius itself, channeling it through the sun. Therefore, when an arrow is loosed from the bow, it produces a magical effect very similar to being burned by fire. But that's only a fraction of its potential.

With Sunhallowed Arrows, you would be able to produce a much more spectacular effect... causing bursts of sunlight to envelop your foes. The sunbursts would certainly hurt anything, but is especially devastating to the undead.

Using an arrow with the bow that's been dipped in blood may cause it to function differently, corrupting its purpose.

Ask Gelebor how to obtain Sunhallowed Arrows. He can assist you in that regard. If you were to bring him some good quality elven arrows, he could imbue them with the proper incantations and rituals.

Give him the elven arrows you have, and he makes them into Sunhallowed Arrows

Where to find Elven Arrows?
You get 12 Elven Arrows for completing "Touching the Sky"
You can also buy them in various stores. Check with the Fletcher west of the Solitude Gate, he usually has a supply of them.
Enemy elves such as Thalmor Soldiers carry them
Other than that, you can find them in certain dungeons.

When you are ready to proceed, speak to Serana.