Prophet (Dawnguard)

Background
''Serana told me that her father was trying to gain power over the sun by fulfilling a prophecy found in the Elder Scrolls. If we could read the prophecy, we might be able to stop him. Serana had one of the scrolls, but we'd need a Moth Priest in order to read it.''

Ask a Carriage Driver
Ask any carriage driver outside a city if he has seen a Moth Priest. He will mention that he's not sure, but can be persuaded or bribed for answers. The driver will reveal that the Moth Priest asked him to take him to Dragon Bridge. If you have the Hearthfire Addon installed, you cannot use your own personal carriage driver.

Ask an Innkeeper
Ask any innkeeper in any city if they have seen a Moth Priest. They will reveal that the Moth Priest had recently stayed in the inn, on his way to the Dragon Bridge.

Visit the College of Winterhold
Head to the College of Winterhold, and speak to Urag gro-Shub in the The Arcanaeum. He will reveal that a Moth Priest came by to study in the library, but left for Dragon Bridge.

Dragon Bridge
Upon reaching Dragon Bridge, ask the guards or Clinton Lylvieve about the Moth Priest's whereabouts. They will mention that he left south across the bridge a short while ago.

Travel south across the bridge and eventually an ambushed carriage can be found. Search the vampire corpse for a note indicating that the Moth Priest was taken to Forebears' Holdout.

Forebears' Holdout
Upon entering the cave, head left and kill the Death Hounds and vampires before crossing over to the main building and up the stairs to the upper platform. The Moth Priest, Dexion Evicus, is being held here within a magical barrier.

Once all the vampires and dead, search the corpse of Malkus to find a Weystone Focus. Place the Weystone on the altar above the barrier to deactivate it. Dexion Evicus will awaken, but will still be partially enthralled. After defeating him, he will come to his senses and thank the Dragonborn. After talking to Dexion, he will agree to help read the Elder Scroll and will head to Fort Dawnguard.

Travel back to Fort Dawnguard and report to Isran. Next speak with Dexion, who will begin to read the Elder Scroll. He first sees a vision of Auriel's Bow, then a voice whispers to him about a dread lord of the night who will rise up in a time when dragons return. He then reveals that two other scrolls are needed to learn the secret of the bow's power and complete the prophecy. One scroll contains the ancient secrets of the dragons, the other speaks of the potency of the ancient blood. Once the reading is complete, the quest ends and Chasing Echoes and Seeking Disclosure begins.

Elder Scroll reading
Dexion: ''"I see a vision before me, an image of a great bow. I know this weapon! It is Auriel's Bow! Now a voice whispers, saying "Among the night's children, a dread lord will rise". In an age of strife, when dragons return to the realm of men, darkness will mingle with light and the night and day will be as one.

The voice fades and the words begin to shimmer and distort. But wait, there is more here. The secret of the bow's power is written elsewhere. I think there is more to the prophecy, recorded in other scrolls. Yes, I see them now... One contains the ancient secrets of the dragons, and the other speaks of the potency of ancient blood.

My vision darkens, and I see no more. To know the complete prophecy, we must have the other two scrolls.''"

Bugs

 * When you go to save the Moth Priest, he may be outside of the magic barrier. If he is, then do not deactivate the magic barrier. If you do, he will stay down after being defeated. Every time you enter Fort Dawnguard or Castle Volkihar, he will attack everyone. The only workaround at this time is loading a previous save.
 * It is possible that Malkar can be inside the forcefield, making it impossible to open without reloading.
 * Upon releasing the Moth Priest, he will be hostile. After he has been defeated, if he does not stand up and remains downed on his knees, then he will attack the other NPCs at Fort Dawnguard or Castle Volkihar when you travel there. The only current way to remedy this is to load a previous save.
 * Solution: You must find the note on a dead vampire on the road south of Dragon Bridge (next to the first destroyed wagon) in order to avoid this glitch. The Moth Priest will still be hostile initially, but will yield upon defeating him.
 * Solution: Get 60 illusion and dual cast pacify frequently. (Only works if you have the perks that work towards calm magic).
 * Solution: Make sure Serana does not follow you into Forebears' Holdout. Even if she accompanied you all the way to Dragon Bridge, as long as she does not go inside with you into Forebears' Holdout, Dexion should no longer be hostile once outside.
 * If on the PC and this glitch is active it can be circumvented by using the console commands KILL and RESURRECT. Press ~ key to bring up the command window. With the console window open use the mouse to click on the the Moth Priest. This will display a HEX code on the window. Type the word "kill" without quotes to immediately place the priest into defeated posture. Exit the console. Once the Priest has completed the defeated posture animation re-open the console. The console should still have the HEX code displayed for the Priest, if not just reselect him. With the HEX code displayed type "resurrect" without quotes. Exit the console. The Priest will then return to normal attach posture, but will react as a friendly until the zone he is in is reloaded. His Compass Marker will still show as an Enemy with a Red dot, but he does not register as an enemy. He does not impeed the use of the Dawnguard forge or fast travel if outside the fortress.
 * Sometimes if Dragonborn is installed and the Moth Priest is hostile and inside Fort Dawngaurd, Riekling Warriors will appear to help (if you became chief at the mead hall in Solstheim).
 * Guards do not run through quest script and cannot be interacted with beyond their usual script.
 * Solution: Bypass the need for speaking to the guards by going south out of Dragon Bridge to find the ambushed caravan.
 * Talking to Clinton Lylvieve will advance the quest and mark the ambushed caravan on the map.
 * Isran's and Serana's script do not activate after following Isran. The only known solution at this time is to reload a previous save.

PC) (Dawnguard) After the Moth Priest reads the Elder Scroll, everytime you try to talk to him when he finishes you will be unable to open dialouge with him. It will appear he is busy and you will not complete the quest and move on the the next.    Prophet (Dawnguard)

Solution : After reading the elder scroll the Moth Priest stays in busy state not allowing you to progress, to fix this open the command console , target him and type disable and right after that without exiting the command console type enable , this should brake the bug allowing you to progress.

PC) If using paralysis poison on the Moth Priest, be careful where he lands. Once he is defeated, he may or may or may not glitch through the floor. He is still possible to talk to, however you must use the console command for No Clip (tlc) to go through the floor yourself to continue the quest.