Kindred Judgment

It serves as the final quest.

Walkthrough
The quest begins with the acquisition of Auriel's Bow, after which Harkon must be confronted. He will be waiting inside Castle Volkihar.

Dawnguard alignment
"For too long we've allowed these vampires to poison the night and kill our people! Now we finally have the means to strike back! We now have Auriel's Bow. The gods themselves have favored us and we must answer with action! The time has come to finally put an end to Harkon and his unholy prophecy! We will march on their lair and destroy those wretched abominations so they can no longer corrupt our world.! This is our fight, and this is our fate. This is the time of the Dawnguard!"

If fighting as a member of the Dawnguard, Isran, Sorine Jurard, Celann, Agmaer, Gunmar and an Armored Troll will join in on the attack on the castle.

The Gargoyles on the bridge will animate and attack. Vampires will pour out of the castle, including Rargal Thrallmaster, Modhna, Namasur, and Garmr. This is one of the few opportunities where firing Sunhallowed Arrows into the sun with Auriel's Bow will provide additional help, provided it's daytime.

Once they are dead enter Castle Volkihar. Inside the remaining vampires will attack, including Hestla, Salonia Caelia, Fura Bloodmouth, Orthjolf, Vingalmo, Garan Marethi, Ronthil and Feran Sadri. Either kill them or proceed into the cathedral for the final confrontation.

Vampire alignment
Upon arrival in the castle's cathedral, Harkon will be waiting and initiate a conversation, which will end in having to choose between 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 from above on the balcony. After his health is depleted, he will retreat to the blood fountain and be invincible while he heals, unless Auriel's Bow is used 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 chose to give him the bow, it can be looted 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 if she can come along on future 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 will be empty.

Bugs

 * Auriel's Bow can vanish from Harkon's remains, meaning the player cannot use its special ability to blot out the sun or use the sunburst attack using the special arrows. If this happens the player cannot gain the achievement "Auriel's Bow."
 * Harkon may walk out of the cathedral, un-hostile, leaving the player behind.
 * Picking up the bow may not start the quest.
 * Halfway through the final battle with Harkon, he may freeze up and become unable to be killed.
 * Serana may become stuck in a battle-ready position at the beginning of the quest, most likely due to the dialogue with the Falmer at the end of the prior quest. Serana can not be spoken with, making it impossible to complete the first step of the quest. A fix is to attack Serana until she drops to her knees, (when she has lost all her health) when she stands back up you can speak to her.
 * The objective to talk to Isran may still be up, even when you have already talked to him, meaning any future speech will not start.
 * Harkon can collapse into ash, but still heal himself, even though he has died. The game will act as if he is still there, and Isran and an Armored Troll will attack his location rather than mention how it's finished. Serana will also attack, meaning she won't wrap up the fight and DLC.
 * Gunmar can be pointed to at Castle Volkihar after the quest, but upon entering may not be there.
 * Despite not being a Dragon, the Dragon battle music will play while fighting Harkon. This might be a bug, or it could mean that he has stats similar to a Dragon.

Trivia

 * If you have the Assassin's Blade perk, Harkon can be killed almost instantaneously while he's still talking. Doing so will make him jump around with his bats a few times and then die.