2. The special power of Auriel's Bow
At the end of the 'Touching the Sky' quest, you receive the ancient artifact Auriel's Bow. This weapon can fire arrows faster than a regular bow and has an enchantment that causes 20 additional points of sun damage, the effect of which is tripled against undead foes such as Draugr and Vampires.
To use the special power of Auriel's Bow you first need to obtain the unique arrows to go with it. These are made by converting Elven Arrows, which are found on Elves or sometimes sold by merchants in major cities, and can also be easily purchased from Sorine Jurard if you joined the Dawnguard.
There are two types of arrow that produce unique effects when used with Auriel's Bow. Sunhallowed Elven Arrows are initially received with the bow, and additional arrows can be obtained from Knight-Paladin Gelebor in exchange for regular Elven Arrows. Bloodcursed Elven Arrows can be acquired from Serana, again by providing her with regular Elven Arrows.
With one of those arrows loaded up, all that's left to do is head outside and fire it at the sun - yes, the sun! A Sunhallowed arrow causes a bright flash in the sky, followed by beams of sunlight that fire down and cause damage to all nearby enemies. Conversely, a Bloodcursed arrow will cause the sun to implode into blackness, plunging the world into night so that Vampires can move around without the draining effect of daylight.