Instructions
Travel to Dawnstar in northern Skyrim. Enter the Windpeak Inn and speak with Erandur. Agree to travel with him to Nightcaller Temple.
Head east and follow Erandur to Nightcaller Temple. Avoid any snow bears you see on the mountain, or take them out with ranged attacks. Erandur will help you in battle by casting destruction magic.
Go to the entrance of the temple. Talk with Erandur once more, and then head inside. Follow Erandur downstairs and take out the orcs. Enter the library.
Kill the Vaermina devotee first when you're attacked by a group of enemies. The devotees use powerful magic, so it's best to defeat them quickly.
Continue through the library. When Erandur stops walking, approach and talk to him. He'll ask you to find an alchemy book in the library. The book's location is marked on your mini map. Once you find it, return it to Erandur.
Follow Erandur to the temple laboratory. You'll need to fight several groups of enemies on the way.
Enter the lab and take the Vaermina's Torpor potion from a shelf. The potion's location is marked on your map.
Return the potion to Erandur. When prompted, open your inventory and drink the potion. You'll enter a dreamlike state that enables you to see and change past events.
Talk to the Vaermina priest in the hall ahead of you. Follow the path to the room marked on your map. You don't have a weapon, but none of the orcs will attack you, so ignore them. When you reach the marked room, pull the chain on the wall to release the miasma blocking your path. This ends the dream and you'll return to the real world.
Pick up the Soul Gem to shut down the magic barrier in the temple's main room. Talk to Erandur and follow him to the main room.
Save your game, if desired, and prepare for a challenging battle with two mages. Drink a lightning-resistance potion to shield yourself from electric attacks. If you're weak to magic, retreat and let Erandur do most of the work, or use a bow and arrows to damage the mages from afar.
Approach Erandur after the battle ends. He'll stand at the altar and perform a ritual to destroy the Skull of Corruption. The evil Daedric lord, Vaermina, speaks to you and encourages you to kill Erandur before he turns on you.
Allow Erandur to complete the ritual and destroy the Skull, or kill him and take the Skull for yourself. If you let Erandur live, he'll offer to be your full-time companion. Either decision ends the quest.