Online:The Skin-Stealer's Lair

Quick Walkthrough
# Enter the Lair of the Skin Stealer.
# Choose whom to aid.
# Enter the vault.

Detailed Walkthrough
Ukatsei thinks he knows where the fake Vicecanon Hrondar is hiding. Earlier, Vicecanon Hrondar ordered his troops to stay away from a nearby hill. There must be an entrance to the temple there. Ukatsei warns the vicecanons while you investigate.

The entrance to the temple is to the south. It is a Daedric portal guarded by Dominion soldiers on a large pavilion at the top of the hill.

Explore the Ruins of Ten-Maur-Wolk
Inside the ruins of Ten-Maur-Wolk and to the right is a Dremora named Lyranth, who is held in place by magical blue chains. She claims she is behind the skin-stealers' abilities. The Dominion summoned her from Oblivion and are siphoning her power.

She offers to tell you the plans of the Dominion alchemist, Ruuvitar, if you set her free. You can agree or refuse.

If you agree, she sends you to find two altars. You need to touch them to remove her bonds.

Release Lyranth
Your first step to free Lyranth is to find the inner altar. However, you can choose to talk to Bijot along the way. He and his friends came to the ruin to look for Daedric artifacts to sell to mages. The Dominion caught and killed his friends, so Bijot is the only one left. He suggests helping each other escape and says that Lyranth cannot be trusted. You can decide to help him by freeing his friends, or you can continue with the Dremora's plan.

Rescue Bijot's Friends
The Dominion captured Bijot's friends' souls in Animus Geodes. You need to destroy three of the geodes to save his friends.

The geodes look like purple crystals surrounded by blue mist and black chains. There is one in an alcove you passed on your way to Bijot, one to the right of the inner altar, and one at the south end of the ruins.

After all the geodes have been destroyed, you talk to Bijot near the inner altar. He says there is a door behind Lyranth where the Dominion are keeping Hist sap. They use the Hist sap to steal skins, but if you burn it you will be able to stop them. Bijot suggests you distract Lyranth while he opens the door to the Vault.

Lyranth reveals that the Hist sap combined with her essence allows the Dominion to create skin-stealers. The Hist sap steals the victim's memories. Ruuvitar's goal is to acquire the Mnemic Egg, along with all of Black Marsh.

The Lair of the Skin-Stealer
Inside the Lair of the Skin-Stealer, you set fire to four vats of Hist sap by interacting with the coals beneath each of them. If you chose to help Bijot, you can talk to him before attacking Vicecanon Hrondar's impostor. Bijot offers to free the impostor's prisoners, as the fake vicecanon will kill them when you approach and gain power from their deaths. Alternatively, Lyranth will offer to capture their souls, thereby killing them before the imposter can utilize them.

You kill Vicecanon Hrondar's impostor, who turns out to be a Khajiit named Sarshar. If you can avoid the deaths of the four prisoners around him by killing him quickly enough, Bijot will stay in the ruins to free the prisoners. He tells you Ruuvitar left to find a place of power where he could use the Mnemic Egg and that the Daedric portal will take you beyond the ruins.

Report to Vicecanon Servyna
Vicecanon Servyna is at the bottom of the steps after you exit the portal. Your actions in Ten-Maur-Wolk allowed the vicecanons to organize an escape from the cave. 

Notes
 
2) with whom you sided with in the quest. 

In case of former: If your character is above Lvl 50, the armor quality reward is blue tier

In case of latter: If you sided with Lyranth, but were quick enough to kill the Khajiit Sarshar the Skin Stealer before he harmed the hostages, the armor reward will be blue instead of green quality. However, the four hostages will still remain bound and cannot be freed or interacted with.

 

Quest Stages