Touching The Sky - p. 4 | Vampire Lord path TES V: Skyrim - Dawnguard Guide
Last update: 11 May 2016
New primary objective: Defeat Arch-Curate Vyrthur
After you finish talking with the Snow Elf, approach his throne and turn your back onto him. It's a good spot for fighting off the incoming waves of enemies. Firstly you will be attacked by giant worms and then by a group of Felmers.
After you defeat them all, the ceiling will start falling down and the enemy will summon a Frost Atronach. Take care of it and chase the crazy priest. You will find him nearby the balustrade, where he will attack you in a more conventional way. The fight shouldn't take too long.
After the fight, speak to Gelebor and pick up the bow from the shrine. it's a really powerful weapon, great against undead enemies. Additionally you can shoot two types of special arrows from it.
In order to create the Sunhallowed Arrows, speak to Gelebor once again. He can change normal elven arrows to such which deal additional sun damage. You need to get them before meeting with Lord Harkon.
Apart from that, Serana can change elven arrows as well. She can create the so-called Bloodcursed Elven Arrows, shooting which cause a solar eclipse for one day. Very useful for every vampire.
Once you have a proper amount of Sunhallowed Arrows, head out to Castle Volkihar. Inside the castle cathedral you will meet Lord Harkon. Regardless of the chosen dialogue options, you will end up fighting the vampire leader. Only the decision regarding whether to hand over or keep Auriel's Bow has an influence on the fight. If you hand it over, you won't be able to use it during the fight. I wouldn't recommend doing that.
During the fight, Lord Harkon will be using a couple skills. Firstly he'll start calling in gargoyles, which you need to get rid of as fast as possible. Apart from that he can also teleport and change into fog (during which he's immune to attacks). Focus on attacking him mostly when he's trying to suck out your life. You can't let him heal too much. The best weapon for this fight is Auriel's Bow with Sunhallowed Arrows. Those arrows are required to break the red barrier which Lord Harkon creates around himself from time to time and additionally deal very large damage to all undead creatures. If you don't have the arrows, you will have to wait for the barrier to disappear itself. The enemy will stop creating it after losing the majority of his power.
When the fight finally comes to an end, the main story of the vampires will come to an end and you character will become the owner of Harkon's castle. Inside the basement you will find people who you can feed on. You can also move to side quests or load the game and start the Dawnguard plot.
Also remember to take the items lying beside the remains of the former Lord.