Kindred Judgment

It serves as the final quest.

Walkthrough
The quest begins after getting Auriel's Bow, after which you must confront Lord Harkon, who will be waiting for you inside Castle Volkihar. If you joined the Dawnguard, they will join you and attack the castle with all their strength, including Armored Trolls. The Gargoyles on the causeway will animate and attack. The Vampires in the castle will attack the Dawnguard. Upon arrival in the castle's chapel, Harkon will be waiting and initiate a conversation, which will end in you being given two options: give him the bow or refuse. Both options result in him becoming hostile. The fight is a standard boss fight, only he will be in his Vampire Lord form.

Lord Harkon will use his Bat's Night power when damaged enough and begin attacking you from above on the balcony. After his health is depleated, he will retreat to the blood fountain and be invincible while he heals, unless you use Auriel's Bow with the Sunhallowed Arrows, which will cause his barrier to vanish. He will summon Gargoyles, so be ready for close range combat. The process repeats itself until he dies with a unique death sequence of a very bloody explosion. If you gave him the bow, you can loot it off his body along with his Vampire Royal Armor and Harkon's Sword. The quest ends with Serana saying a few words about her father and asking you if she can join you on your adventures. If you sided with the Vampires, Castle Volkihar becomes your home with every item in the castle now belonging to you. There are repeatable radiant quests from various vampires in the castle that reward you with leveled equipment and blood potions. If you joined the Dawnguard, all vampires within are slain and the castle is now empty.

Bugs

 * Auriel's Bow can vanish from Harkon's body, meaning the player cannot use its special ability to block out the sun and gain the achievement "Auriel's Bow."
 * Harkon may walk out of the church, un-hostile, leaving the player behind.
 * Picking up the bow may not start the quest.