Skyrim:Unholy Vigil

Quick Walkthrough
#Travel to the Hall of the Vigilant and read Letter to Keeper Carcette.
#Read the Mysterious Note at the Windpeak Inn in Dawnstar.
#Locate Azarain's body at the Windward Ruins and read ''Azarain's Journal.
#(Optional) Pickpocket Harald, Irine, and Vereth to find out which one is a cultist.
#Follow Harald at night and enter the secret chamber within Quicksilver Mine.
#Kill Fenrik and his minions to destroy the Daedric cult.

Detailed Walkthrough
Missing Vigilants
You must read the Letter to Keeper Carcette'' in the Hall of the Vigilant. Note that this location will be destroyed once you reach level 10 or start the quest Dawnguard. If it has been destroyed, the letter can be found underneath a table within the ruined building. Reading this letter will reveal that several members of the Vigil of Stendarr have gone missing, and a Vigilant named Azarain has decided to journey to Dawnstar to investigate.

You must first travel to the Windpeak Inn in Dawnstar to look for clues as to Azarain's whereabouts. A mysterious note in one of the ruins appears to have lured Azarain to the nearby Windward Ruins outside of town. Journey there and you will find his corpse, wearing a full set of the new Vigil Veteran armor which can be looted. You must read his journal to discover that Azarain's old partner Fenrik had come to Dawnstar to investigate rumors of a Daedric cult. Azarain had discovered that the cult uses poisoned Elven daggers to perform their killings, and he had narrowed down his investigation to three primary suspects:  Harald, Irine, and Vereth.

Follow the Clues
Irine can be found wandering around Dawnstar, near the Windpeak Inn. Vereth can be found inside the inn. Harald is a miner and can be found working inside Quicksilver Mine down by the docks. Your journal will suggest that you try pickpocketing each of them to identify which one might be a cult member. You do not need to steal anything to update the journal, and this step can be skipped entirely if you prefer. Instead, you are presented with the option of following each of the three at night to see if they will lead you to the Daedric cult.

The correct person to follow is Harald, who can be found carrying an Elven dagger if you choose to pick his pocket. Wait until nighttime inside Quicksilver Mine, until all of the miners stop working. Rather than exit the mine, Harald will begin to walk deeper inside, before vanishing behind a rock wall. Sneak up behind him and activate the button on the wall to uncover a secret entrance to a large natural cavern.

Irine and Vereth, meanwhile, are innocent: Irine's nocturnal wanderings take her to a shrine of Dibella a little way south of Dawnstar, while Vereth visits the lighthouse northeast of town and stands gazing out over the sea.
thumb|left|The blood altarthumb|right|Grim loot found within
Temple to Molag Bal
This cavern, it is revealed, has been converted into a temple to Molag Bal. It contains a large central blood altar surrounded by coffins, many of which are filled with dead Vigilants of Stendarr. You must kill Harald along with the vampire Fenrik and his three other thralls. Killing all of the enemies will complete the quest, although ''Fenrik's Journal'' goes on to explain what had happened to lead to these disappearances. It reveals that Fenrik had become infected with vampirism and had sought a cure, which was blasphemous in the eyes of his fellow Vigilants who believe that vampires are unredeemable monstrosities. To cure his disease, Fenrik petitioned Lord Molag Bal to lift the curse, and in return Bal demanded that he sacrifice nine of his fellow Vigilants (one for each of the  ).

Fenrik's body can also be looted for a full set of Vigil Corrupted armor. A set of Vigil Enforcer armor can be looted from the corpse of one of the sacrificed Vigilants. Finally, a set of Vigil Silver Hand armor can be found lying on a table on the northeastern side of the cavern, next to the bloody red bones of the previous wearer. All four of the new armor sets can also be crafted at a forge once you've completed this quest, provided you have the Steel Smithing perk.
 
Notes
The other two suspects will vanish from town once this quest is completed.
''Galur Rithari's Papers can be found within the cavern, on a table to the south of the altar. This rare book was needed in Morrowind'' to cure yourself of vampirism by performing a task for Molag Bal.

Bugs
 
 

Quest Stages