Vampire Faction -- Prophet
While the role that the Chalice will be playing in the ongoing process is not clear at this point, the return of his daughter for Harkon also marks the return of the Elder Scroll that she carries -- but the problem with that is that the vampires do not have anyone in their ranks who can actually read it -- and that means you will be tasked with helping to correct that temporary deficiency. It just so happens that while the vampires do not have a scholar who is proficient in the language of the Elder Scrolls, the Dawnguard do! You know what that means, right?
He tells you about the Elder Scroll and the prophecy that he learned of, and provides you a bit more of the background and then Harkon addresses his people, telling them:
"Scions of the night! Hear my words! The prophesied time is at last upon us. Soon we will claim dominion over the sun itself, and forge a new realm of eternal darkness.
"Now that I have reclaimed one of my Elder Scrolls, we must find a moth priest to read it. I have spread false rumors about the discovery of an Elder Scroll in Skyrim to lure a Moth Priest here.
"Now it is time to see if those efforts have borne fruit. Go forth, and search the land for rumors of a Moth Priest within our borders. Look to the cities. Speak to innkeepers, carriage drivers, anyone who would meet a traveler. Go now, and carry out this task. This is my command!"
An En-Thralled Moth Priest
-- The Quest to find a Moth Priest --
It works out that Serana chooses to accompany you and tells you so -- go ahead and exhaust the chat options that she presents you with as that will allow you to obtain more background information about her and her family and how they became what they are. As odd as it sounds and as friendly as she is with you (meaning as non-suck your blood kill you now as she was when you first met her) she really does not view being a vampire as a bad thing.
Assuming that you are ready, and you have everything that you think you need for adventuring, it is time to depart the castle. Note that as you head out you get the warning that your vampire blood is boiling -- you may want to feed on a Thrall before you leave, since you could easily be at Stage Three and at least Stage Two by now... Just saying... The human cattle are in pens in the cellar below the kitchen right off of the main dining room so head there and feed before leaving to make your life less exciting, right?
It is suggested that you begin the search at the Mages College -- but you do not have to do that, you can cut to the chase and head for Dragon Bridge where you will talk with either a guard to learn that you just missed him -- he left town across the Dragon Bridge. So chase after him!
Following the road south you will come upon a wagon that has been attacked and knocked on its side. Search the bodies here to find the Vampire's Note -- which instructs the vampires to attack the wagon and take the Moth Priest to Forebears' Holdout. Right then, we now know where the bugger is!
Check the map and you will see that the destination is not too far away - head there and you will find as you approach that there is evidence of a battle outside -- blood trails and kill spots marked by blood! Clearly something is not right here.
When you reach the back section you are attacked by Dawnguard soldiers - what are they doing here?! A more complete search reveals that the Moth Priest is gone and up top you find a vampire named Malkus -- or rather the corpse of a vampire named Malkus -- and when you search him he has a key item called a Weystone Focus that you need to loot.
In the center of the chamber is a whirling force field and inside of it, partly concealed is the Moth Priest! Excellent! Place the Weystone on the altar at the top of the stairs to deactivate the force field and you will then need to subdue the Moth Priest -- who it seems does not much care for vampires -- which remember you are!
It turns out that the Moth Priest has a name -- and it is not Moth Priest! It is Dexion Evicus, and with him subdued you need to cast Vampire Seduction on him (make sure you aim well because that power can only be used once a day) and then bite him (feed on him like you would a Thrall - you know, go into Sneak and bite from behind) to turn him into your Thrall, at which point he will do anything that you tell him.
So tell him to walk to Castle Volkihar and then go there yourself and report in to Harkon, who is very pleased with your success. It seems like you are turning out to be a most excellent member of his crew -- why, he is as happy as Tony Soprano after his HUD scam!
Harkon orders you to command your new Thrall Dexion to read the Elder Scroll and reveal all that it says -- and he does -- but it appears that this is not all of the information that is required to make use of the knowledge that Harkon already has and blot out the sun...
The Moth Priest tells you about Ariels Bow and a story-puzzle of sorts, but he needs all three of the Elder Scrolls in order to complete the prophecy! When you speak to Harkon you learn what he knows about the other two scrolls -- well, his missing wife has one, but we know where the other one is, don't we!
That completes the quest PROPHET and triggers the quests CHASING ECHOES, SEEKING DISCLOSURE, and SCROLL SCOUTING. Well done mates!