Prophet (Hunters)

Walkthrough
After having recruited two new valuable members for the Dawnguard the question is how to take on Lord Harkon and his clan of vampires.

Follow Isran
Apparently fate is with the Dawnguard, since a vampire has been "captured" and an "interrogation" should yield important information.

Speak to Serana
Well, the vampire is Serana, and she has come to Fort Dawnguard of own her free will, and the interrogation is actually a conversation among trusted friends.

She mentions that her father, Lord Harkon himself, has long gone insane, obsessing with an ancient prophecy found in an Elder Scroll to overcome the "tyranny of the sun", plunging the land into eternal darkness. Serana, therefore offers her services to the Dawnguard to stop Harkon.

Speak to Isran
Isran does not trust Serana one bit, and will hold the Dragonborn responsible for her actions, but grudgingly agrees to her help. The first step is to decipher the Elder Scroll Serana has brought with her, but only a Moth Priest would be able to read it.

Locate a Moth Priest
So such a priest must be located first.

Ask carriage drivers about the Moth Priest (Optional)
There are several possible paths of inquiry. One path is to ask the carriage drivers located outside most of the larger cities in Skyrim. E.g. try the one at the Windhelm Stables. The carriage driver may require some persuasion or 837 though to divulge information.

Ask innkeepers in cities about the Moth Priest (Optional)
Alternately, asking around for a Moth Priest recently seen by innkeepers should also help track one down.

Visit the College of Winterhold to ask about the Moth Priest (Optional)
Or, as suggested by Serana, ask around at the College of Winterhold, i.e. visit the librarian Urag gro-Shub there. He will mention that recently a Moth Priest visited the library, and then continued on to...

Ask people in Dragon Bridge if they saw the Moth Priest
Any one of the methods will finally direct the Dragonborn to Dragon Bridge, where a quick chat with the boy, Clinton, reveals that the heavily guarded Moth Priest actually did pass through the small town on a carriage, leaving in south-ward direction.

Search along the road south of Dragon Bridge
Heading south the Dragonborn soon comes to the scene of an overturned carriage, where a fight seems to have taken place between Imperial guards and vampires.

Investigate the scene of the attack
This may well have been the wagon the Moth Priest used to travel. On closer inspection one of the dead vampires carries an interesting Vampire's Note.

Read the Vampire's Note
Reading the note uncovers that another group of vampires has kidnapped the priest and has taken him to their lair in Forebears' Holdout.

Note: This location will not be unlocked on the map yet, since it only appears in this quest.

Rescue the Moth Priest
Entering the Forebears' Holdout, defeat the vampires. Apparently Malkus has enthralled the priest, so he must be defeated first. Then take the Weystone Focus from Malkus's body to unlock the Weystone Source field and thus free the captive priest.

Defeat the enthralled Moth Priest
The enthralled Moth Priest immediately attacks and must be beaten into submission.

Speak to the Moth Priest
Dexion Evicus, the name of the priest, will then thank the Dragonborn for freeing him from enthrallment. And once hearing of Serana's Elder Scroll he will gladly read it, and visit Fort Dawnguard to help the Dawnguard there.

Report your success to Isran
Return to the fort, and report the success of the mission to Isran.

Bugs

 * A too efficient killing of the vampires in the cave may lead to a bug where Dexion can actually escape the field without shutting it down first. Plus the priest may start attacking Serana after exiting the hideout. Later at Fort Dawnguard the priest may even attack Isran and then fail to read the Elder Scroll. Only way to fix this problem, is to fall back to an older save state, re-enter the Forebears' Holdout, and replay the rescue again.

Prophet (Jäger)