Daedra God: Vaermina
How to trigger: To start this quest, first head to the major city of Dawnstar. Once you're there head inside of Windpeak Inn. Inside you'll here a group of citizens complain about nightmares. Go listen in and you'll learn that everyone in the town has been having re-occurring nightmares. After the priest Erandur gets everyone to calm down, go talk to him. He'll state that in fact, the source of the nightmares is likely coming from a place called Nightcaller Temple, a place of worship for the Daedric Lord Vaermina. He will ask for your help in putting in end to it. Agree to start the quest.
We'll now have to follow Erandur to Nightcaller Temple, so follow him as he leads the way outside of the Inn. He'll take you up to the Nightcaller Temple, a short walk up a mountain outside of town. Beware of the Ice Trolls near the top. Once you get there, Erandur will explain the miasma in the place and the orc party that attacked years ago. After hearing that, follow him into the temple.
In here he will show you the Skull of Corruption, an artifact that Erandur believes is eating memories. We have to get down to where it is and perform a ritual. Head down the stairs and two orcs will awaken. Take them out. The way forward is blocked, and so Erandur will say we need to head to the library (you can find out how he knows so much about the place now...). Head up to the library and clear out the priest and orc at the top, and two more priests at the bottom. Erandur will state we need to find a book called "The Dreamstride" now, and a marker will appear to show you where in the room it is (upstairs across a fallen stone column).
Go grab it and return it to Erandur, who will flip through it and deduce we need a liquid called Vaermina's Torpor so we can use an ability called a Dreamstride... just listening to this makes me nervous! Also, go find a chest behind a door here on the lower level before following Erandur to the lab. You will have to fight two more enemies in an open room before going into the lab where FIVE enemies will wait for you. Take them all out (hit them as they get up) and Erandur will state we need to find the Torpor.
Once again the marker makes it easy to find the Torpor, but be sure to raid the room for the abundance of alchemy ingredients and an ALCHEMY SKILLBOOK called "Mannimarco, King of Worms". Once you're done looting, grab the Torpor and show Erandur. He'll tell you to drink it (it's in your inventory under potions) so do so. Follow out the dream sequence (you can't grab stuff or attack anyone) and release the miasma. After you do so, you'll "wake up" in the real world and you'll be BEHIND the barrier.
Take the soul gem to open the path for Erandur and continue on. Kill anyone that awakens together and be sure to look out for chests and potions. At the end of your journey (down near the skull) you'll have to face the two priests you saw in the dream (and you'll learn who YOU controlled for a time). Take them out before they kill you and loot them. There's also a chest in the right hand cubby down here. Erandur will now start the ritual to remove the barrier. As he does so, you will HEAR Vaermina tell you that he's going to betray you, and you should kill him and take the skull of corruption for yourself. This of course gives us two choices...
Note: Keep in mind that for the 'Oblivion Walker' trophy, you NEED to take the Skull of Corruption.
o Let Erandur destroy the skull:
Don't do anything! Let Erandur finish his ritual! He will end up destroying the skull. Afterwards, talk to him and you will complete this quest. The "reward" by taking this path is that you'll be able to take on Erandur as your companion whenever you would like. He will reside at Nightcaller Temple should you need him.
o Kill Erandur and take the skull:
To take this path, attack Erandur as he is performing the ritual. Keep attacking him and slay him (a sneak attack is VERY easy to pull off here). Be sure to loot him afterwards and claim the Skull of Corruption as your own. The skull is a staff. It does 2 damage but is enchanted with this effect: "20 points damage, increase to 50 if powered with dreams collected from sleeping people." Once you claim it this quest will be over.
Note: When the staff says it increases its damage to 50, that's IF it has collected dreams. To collect dreams, you must find a sleeping person and USE the staff on them. This will steal FIVE dreams from them. However, it will also awaken them right then and there, so be careful.