Adventures in Skyrim - spoilers ahead! You can follow the rest of the story here!
While Njada and the rest of the Dawnguard secured the castle, Serana and I headed up the last tower to the cathedral where we found Lord Harkon waiting for us. He has a few choice words for his daughter and she for him which distracts him long enough to not notice me sneaking behind his back. When he finally realizes I'm missing, I hit him with a sunhallowed arrow and commence the battle. He manages to survive for awhile, prolonging the fight with summoned scum but in the end, he is turned into a pile or red dirt on the floor.
Once he is dead Njada and Isran enter, the latter thanking Serana for her help and the former just happy to see me ok. As we are leaving, I convinced Serana to find a cure for her vampirism and after thanking me she headed off on her own to pursue that personal goal. Provided she is successful this would just left one more vampire in her house, her mother. After making a quick trip into the Soul Cairn once again I found Valerica at the boneyard and executed her on the spot. Rather, I would have liked to if she wasn't protected by threads of fate (DAMN GAME).
Instead I tell her Harkon is gone and she returns to the castle to pursue her studies. In necromancy. As a vampire. This riles me up pretty bad, but there's not much I can do about it right now so I make like the Terminator and tell her "I'll be back". After a quick stop to Markarth to sell Calcemo an exploding dwarven bolt, I take Njada to visit Fort Dawnguard since she wanted to have a look before heading off to her new residence at Windhelm. While there I agree to help out with a few loose ends - more vamps on the loose whose locations were found from the castle tomes. The elf Beleval is assigned this patrol with me and her skills are tested early as we ran into (and slew) a pair of vampire scouts just outside Fort Dawnguard's walls.