Follow Isran, who leads you to Serana, who is waiting in a small room inside the fort. He guesses this is the woman you found in Dimhollow Crypt. She has something really important to say to you. Speak to Serana. It is about her, and the Elder Scroll that was buried with her. It all comes out back to her father, Harkon. Harkon stumbled onto this obscure prophecy, and just kind of lost himself in it. A prophecy which would let Harkon control the sun, and have vampires control the world. Serana and her mother did not want to start a war with all of Tamriel, so they tried to stop him. That is why Serana was sealed away with the Scroll.

After talking to Serana, speak with Isran again. He heard what Serana had to say, but questions her motives. He asks you if there is any reason he shouldn't kill this bloodsucking fiend right now. Tell him you might need her help. But Isran doesn't believe her story about the prophecy and about some vampire trying to put the sun out. Tell him she had no reason to risk her life coming here. Eventually, Isran very reluctantly agrees to let her help.

Serana points out that she has the Elder Scroll with her. But neither she or you can read it. Moth Priests are the only ones she has heard of who can do it. They spend years preparing before they start reading, though. But they are all half a continent away in Cyrodiil. But Isran mentions that some Imperial Scholar arrived in Skyrim a few days ago and suspects it was a Moth Priest. But Isran does not know where the Moth Priest is staying now. He suggests you talk to anyone who would meet at traveler, including innkeepers or carriage drivers in big cities.

You get the following objectives:
Locate a Moth Priest
(Optional) Ask Carriage Drivers about the Moth Priest
(Optional) Ask Innkeepers in Cities about the Moth Priest
(Optional) Visit the College of Winterhold to ask about the Moth Priest

Before heading out, you can ask Serana if she was always a vampire. Vampirism came from a Daedric Lord. The first vampire came from Molag Bal. She was not a willing subject, but she was still the first. Molag Bal is a powerful Daedric Lord, and his will is made reality. For those willing to subjugate themselves, he will still bestow the gift, but they must be powerful in their own right before earning his trust. The ceremony in which Serana became a vampire was degrading. Eventually her father became unstable, driving her mother crazy with him, and ending with Serana being locked underground for who knows how many years.

Exit Fort Dawnguard, and start searching for the Moth Priest.