Waking Nightmare

Walkthrough
This quest is the start of becoming thane of Dawnstar. The Dragonborn can hear about it in various ways. There is a letter on the floor of Aerin's House in Riften which pleads with Mjoll the Lioness to help the residents of Dawnstar with their nightmares. Alternatively, the Dragonborn can speak to the Jarl of Dawnstar, or to Erandur directly.

Speak to Erandur about the Nightmares
Erandur can be found in the Windpeak Inn, at the bar. After asking him about the nightmares, the Dragonborn can decide to help him.

Note: There may be a minor bug whereby Erandur will not leave his position at the bar to begin the trek to Nightcaller Temple. One way around this is to use the Unrelenting Force shout to blast him towards the door. After he picks himself off the floor, he will set out for the temple and the quest can begin.

Follow Erandur to the Nightcaller Temple
If the Dragonborn has a follower, they will not be able to enter the temple, only Erandur and the Dragonborn may enter. The Dragonborn then simply follows Erandur as he explains the history of the events at the temple. He explains that the nightmares are being caused by the Skull of Corruption, but that the skull is currently inaccessible because of a magical barrier.

Follow Erandur to the Library
Once in the library, the book The Dreamstride needs to be located, which is on a pedestal on the upper level.

Find and Drink the Vaermina's Torpor
The Dragonborn continues following Erandur to the laboratory where there is a bottle of Vaermina's Torpor. Erandur then tells the Dragonborn to drink the mixture which sends the Dragonborn into "the Dreamstride" as the priest watches.

While Dreaming: Release the Miasma
Inside the Dreamstride, the Dragonborn sees the world through Brother Casimir's eyes (later it is revealed that Brother Casimir is Erandur's former name). In the dream state, the Dragonborn then makes way back towards the entrance of the temple where there is a pullchain which releases the miasma.

Breaking the Barrier
Once the chain has been pulled, the magical barrier preventing access to the Skull of Corruption is dispelled. Erandur then confronts two of his former friends, Veren Duleri and Thorek, that he thought he had left for dead. He and the Dragonborn then kill them and proceed to the skull where Erandur begins his ritual.

At this point, Vaermina tries to persuade the Dragonborn to kill the priest before he can complete the ritual.

Option 1: Kill Erandur
If the Dragonborn kills Erandur, the Skull of Corruption, a powerful staff, can be taken.

Option 2: Let Erandur Destroy the Skull
If the Dragonborn ignores Vaermina, Erandur finishes his ritual which destroys the staff, but then offers his services as a follower.

Ein lebendiger Albtraum