Prophet (Vampires)

Walkthrough
After having filled the chalice in The Bloodstone Chalice and having returned to the castle, Garan Marethi directs the Dragonborn to Lord Harkon.

Speak to Harkon
Visit him in his chambers.

Listen to Harkon's speech
He plans to end "the sun's tyranny" over the vampires using a prophecy found in the Elder Scrolls. But only a specially trained Moth Priest can read them.

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 spill the beans.

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.

Interestingly, asking the innkeeper Dagur at the The Frozen Hearth in Winderhold may unearth a rumour of a mage who can cure vampirism, launching the quest Rising at Dawn.

Visit the College of Winterhold to ask about the Moth Priest (Optional)
Or, as suggested by Serana, ask around at the College of Winterhold.

Speak to the innkeeper in Solitude
Another rumour may lead to Solitude, where the Moth Priest seems to have stayed at The Winking Skeever inn. Ask the innkeeper about this.

Ask people in Dragon Bridge if they saw the Moth Priest
Any one of the methods will finally direct the Dragonborn to Dragon Bridge, were 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.

Capture the Moth Priest
Entering the Forebears' Holdout, defeat the Dawnguards and an Armored Frost Troll. Apparently Vanik has enthralled the priest, so he must be defeated first. Then take the Weystone Focus from Vanik'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.

Use your Vampire's Seduction power on the Moth Priest
Using Vampire Seduction on the priest calms him down, then feed on him to turn him into the Dragonborn's thrall. The name of the priest is Dexion Evicus.

Command the Moth Priest to go to Volkihar Castle
Then command him to travel to Castle Volkihar. He will immediately comply, after the Dragonborn explains how to get to the castle.

Report your success to Harkon
Return to Castle Volkihar and report on successfully completing the mission.

Command the Moth Priest to read the Elder Scroll
Give the priest the Elder Scroll (Sun) (from Serana) and make him read it.

Speak to Harkon
As it turns out there are two more scrolls that need to be found, one Elder Scroll (Blood) is in the possession of Serana's mother and Harkon's wife Valerica, the other Elder Scroll (Dragon) may already be in the Dragonborn's possession.

Thus begins the hunt for the other scrolls in Chasing Echoes and Seeking Disclosure.

Prophet (Vampire)