As you approach the Castle Volkihar gates, the Watchman stands watch on the other side of the gate. He tells you that you are not welcome here. Serana wants to talk to you before you enter the castle. Ask her what's on her mind. She wants to thank you for getting her this far. But after you get in there, she is going to go her own way for a while. She knows that your friends in the Dawnguard would probably want to kill everything in the castle. She is hoping that you can show more control than that. She asks that once inside, you should keep quiet for a bit. Let her take the lead.

The watchman notices Lady Serana is back and gives the orders to open the gate. Enter Volkihar Keep. Serana tells you this is her home.

Vingalmo shouts at you, how dare you trespass here. But suddenly he sees Serana and calms down. You have completed the objective of escorting Serana to her home. Your new objective is to talk to Lord Harkon.  Enter the dining all. In addition to Harkon, there are other NPCs here such as Feran and Hestla. Harkon is happy to see his long last daughter returned at last. He asks her if she has his Elder Scroll. Serana is upset that after all these years, that is the first thing he asks her, but she tells him that yes, she does have the Elder Scroll. He wishes her traitor mother was here.

Harkon asks Serana about the stranger she has brought into their hall (you). She tells him that you are her savior, the one who freed her. Harkon tells you that you have his gratitude for his daughter's safe return. He asks you what your name is. You tell him, and he tells you that he is Harkon, lord of this court. He assumes Serana has told you what they are.

Your choices are:
You're vampires
You're a reclusive cannibal cult
I don't really care what you are.

If you accuse them of being a reclusive cannibal cult, he tells you that not quite, but he can see how an outsider might arrive at that conclusion. They are vampires, among the oldest and most powerful in Skyrim. They have lived here for centuries.

Next, you will have a very important choice to make.