This is the final quest in Skyrim Dawnguard on the Dawnguards side:

Kindred Judgment
With Auriel's Bow in my possession, I need to rally the forces of the Dawnguard before I attempt to confront Harkon and put a stop to the prophecy

Confront Harkon with Auriel's Bow.

Fast travel to Castle Volkihar. Isran, Sorine Jurard, Celann, Agmaer, Gunmar, and any other Dawnguards alive at this point, are waiting for you near the castle. Storm the castle, killing the Gargoyle Sentinels along the way. Other Vampires will also be there to protect their castle, including Modhna, Namasur, and Stalf. Garmr, a death hound, will also attack you as you approach the castle. There is a Rargal Thrallmaster attacking your party from near the entrance, kill it as well. Enter Castle Volkihar.

Inside you will run into more vampire enemies including Hestla, Salonia Caelia, Fura Bloodmouth, Orthjolf, Vingalmo, Garan Marethi, Ronthil, and more.

Follow the marker on your compass and eventually you reach a gate, activate the pull chain, then enter the Volkihar Cathedral. Talk to Lord Harkon inside. Harkon demands you turn over Auriel's bow to him. Do not agree.

Lord Harkon Decision - What happens if you give Lord Harkon the bow?
If you give the bow to Harkon he will betray you and attack you anyway (he will say you are pitiful and that you he expected more from you) and you will be in a disadvantageous position in that combat since you will not be holding the bow anymore.

How to Kill Lord Harkon

Adds: Lord Harkon summons Gargoyles, Gargoyle Brutes, and Skeletons to fight against you. Have Serana deal with those, while you focus on Harkon himself

Teleporation: Lord Harkon will frequently become a swarm of bats, teleporting from one place to another. If you are a ranged character, stay at a strategic position where you have a line of fire to most of the chamber. Otherwise, chase Harkon as soon as he teleports to his new location, and engage in melee.

Weapon: Use your best weapon, not necessarily Auriel's Bow (see next bullet)

Auriel's Bow: If you have a LOT of Sunhallowed Elven Arrows (40 or more), use this bow freely during the fight. However if you have less than 40 arrows, you should only use the bow when Harkon is regenerating in the Blood Shield

Blood Shield: When Lord Harkon's health becomes low, he teleports to the altar and raises a red shield around him. This red bubble is IMPENETRABLE to normal weapons. Use Auriel's Bow, armed with the Elven Sunhallowed Arrows to breach his shield.

If you gave Auriel's Bow to Harkon
If you gave the bow to Lord Harkon, or if you are out of Sunhallowed Elven Arrows, you can not penetrate the blood shield. Wait for Harkon to recharge (help Serana with the adds in the meantime) and attack Harkon again as soon as the shield goes down. Harkon will use the blood shield two or three more times before he runs out of power, so be patient and you will kill him eventually. If you have Auriels Bow and the Sunhallowed Arrows than you can kill Harkon faster.

After you have killed Harkon
Loot his body for some good stuff including Royal Vampire Armor, and Harkon's Sword. If you had given Harkon Auriel's Bow, you will be able to loot it back now.

You can ask Serana to make you into a vampire. This will transform you into a Vampire Lord, as if you had accepted Harkon's Gift at the beginning.

Serana tells you she will stay with the Dawnguards, but she can be your follower if you want.

Congratulations, this completes the Dawnguard Questline on the Vampire Hunter Path!