Vampire Faction -- Kindred Judgement
The Elder Scrolls V: Skyrim - Dawnguard Walkthrough and Guide
by CMBF 
Vampire Faction -- Kindred Judgement
When you return to the castle you are confronted by Harkon who demands you make a choice to either give him the bow or refuse -- you refuse and the battle ensues! For his part Harkon will fight in Vampire Lord form, you do as you like.
Whenever he is weakened badly he will go to the blood fountain to heal -- and during these healings he is invulnerable unless you use the bow and the special arrows to smash the force field around him, at which point he can be attacked, but expect him to summon Gargoyles to defend him. This process will repeat until you kill him, so it would be an idea not to dawdle and kill him as quick as you can, right?
The Boss Battle with Harkon
After the battle ends you can loot his Vampire Royal Armor and Harkon's Sword -- and if you plan on remaining a vampire you probably should do that?
After you win Serana asks if she can be your companion -- and the Castle is now your home. Everything in it pretty much belongs to you. There are two additional quests left for you to do after your victory -- talk to the usual quest givers in order to get them (you may have to do some of the random quests before you get them).
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Having chosen this route to play. I had myself cured of Vamperism. With Serena (Pronounced Sarana) at my side we were continuously attacked by members of the DawnGaurd. I eventually parted ways with Serena. Made sure that I was not infected with Vamperism. Yet I still get the random attack from members of the DawnGaurd, even though I am not a Vampire or travelling with a Vampire.