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Background

You'll receive this quest at the conclusion of the quest Touching the Sky.


Walkthrough

When you talk to Serana after obtaining Auriel's Bow, one of two things will happen:

• If you sided with the vampires, then Serana will convince you that Harkon has to die or else you'll never be safe. You'll then need to travel to the cathedral inside Volkihar Keep to confront him. Nobody will attack you on your way there.
  
  
• If you sided with the Dawnguard, then you'll need to travel back to Fort Dawnguard, where Isran will decide that now is the time to attack the vampires. He'll then send all of the Dawnguard with you to Volkihar Keep, where you'll need to rout the remaining vampires. After that, you'll need to enter the cathedral, where you'll find Harkon waiting.

When you meet with Harkon inside the Volkihar Cathedral, he'll already be in vampire lord form, giving you a clue about how the encounter will go. But when you arrive, he'll start a conversation with you, and he might even ask you to hand over Auriel's Bow. However, no matter what you say or do, he'll end up attacking you, so it's better to hang onto the bow. You can also just attack Harkon when you arrive, if you want to skip the conversation.

When Harkon attacks you, he'll do a few things: he'll summon skeletons, he'll summon gargoyles, he'll cast drain life on you, and he'll generally race around the cathedral staying away from you -- sometimes even transforming into a swarm of bats to escape. Then, once his health has dropped low enough, he'll flee to the altar at the western end of the room, and he'll put up a shield while he regenerates his health. If you have Auriel's Bow, then you can shoot the shield (with any arrow) to disrupt the process and get Harkon fighting again. Otherwise, he'll only return to combat once he's regained his health. If you're quick with the bow, then you'll save some time in the battle as you won't need to "re-damage" Harkon as much, but no matter what, Harkon will probably head to the altar two or three times before you'll be allowed to finish him off.

After the battle, when you loot Harkon's ashes, you'll find Harkon's Sword, Vampire Royal Armor, and more. Serana will then talk to you, and you'll be able to keep her as a companion or not. If not, then she'll stay in Volkihar Keep (if you sided with the vampires) or she'll go to Fort Dawnguard (if you sided with the Dawnguard). You'll also be able to return to your faction and complete some more side quests, if that sort of thing interests you.