Skyrim:Siege on the Dragon Cult

Quick Walkthrough
# Speak to Captain Valmir to begin the quest.
# Enter the ruins of Forelhost.
# (Optional) Find the journal of Skorm Snow-Strider.
# Make your way through the first zone into the Crypt.
# Either pick the lock on the well or find the key, then move on to the Refectory. 
# Travel through the Refectory, find the glass claw, and locate the puzzle door.
# Use the claw to enter and defeat Rahgot.
# Return to Valmir with the mask.

Detailed Walkthrough
thumb|right|Valmir's camp sits in the ruins of Forelhost
When you enter the courtyard of Forelhost for the first time, Captain Valmir will call to you. When the two of you get close enough, he'll initiate a conversation. He will claim he has been sent there by military forces (Imperial or Stormcloak, depending on either your allegiance in the civil war or which soldier you followed in Unbound), in an attempt to recover a powerful artifact for the war effort. He asks for your help, since he cannot do it alone. According to him, this was the last stronghold of the Dragon Cult back in the First Era, and the commander's mask was allegedly an extremely powerful artifact. He requests that you enter the ruin and retrieve the item. He also mentions that the military general who stormed the fortress to wipe out the cult supposedly left his journal here, so finding it might provide some more information. 

Forelhost Stronghold
Shortly after entering the zone, you'll begin encountering ghosts of the cultists. They must be defeated or avoided to proceed. Work your way around to get behind the portcullis that is down, and you'll be able to enter the crypts. ''Skorm Snow-Strider's Journal'' is found along this path. It recounts that when they took the fortress, the defenders had collapsed the entry and all killed themselves, leaving the ghosts to haunt the place. The well was locked and poisoned, and with the losses suffered from the poisoned water, Skorm decided not to search for the key in the crypts, simply choosing to leave the survivors sealed within.

Forelhost Crypt
Once in the crypt, make your way to the well. If you can pick the lock, do so, and move on down; otherwise turn right instead and head through the crypt tunnels to the far end, where you'll find the key. Then head back to the well. After going down the well, follow the tunnels on to the Refectory. You will find a potion on top of a stand with dart holes behind it - you can drop a wooden plate or something similar on the ground, pick up the potion, and use the plate/object to stop the darts.

Forelhost Refectory
Once in the Refectory, move through and find the glass dragon claw, which you'll need to open the puzzle door later. Continue on to the door. From top (outer ring) to bottom (inner ring), the solution is: Fox&mdash;Owl&mdash;Snake.

You'll need to defeat the dragon priest Rahgot and four leveled draugr. Afterwards, loot Rahgot's body. He'll have the mask, a dragon priest staff, and the key to the battlements.

Battlements
Head out onto the battlements to reach a word wall, and learn a word for the Storm Call shout.

After visiting the word wall move near the inside of the battlements; you'll see Captain Valmir talking to an enemy soldier. The objective to return to Valmir remains, so drop down. You'll find that Valmir has now dressed in the armor of the opposing side, to convince this other soldier to do the same job he sent you to do! He's an impostor! Defeat both Valmir and the enemy soldier to conclude the quest. If you wish, loot ''Valmir's Orders'' to discover that he was actually an agent (strongly implied, but not explicitly stated, to be Thalmor) who was sent to retrieve the mask and return to Labyrinthian with it (presumably to use the mask on the matching pedestal in Bromjunaar Sanctuary).

Notes
It is possible to ignore Valmir at the beginning of the quest (by ending conversation before agreeing to take the quest) and pickpocket the key to the ruins from him. You'll get an entry in your journal about his quest only when you reach the battlements and overhear his conversation with the second NPC.
It is also possible to kill Valmir immediately after arriving at this location but before going into the ruins. You can then loot his body for the scrap of paper revealing his treachery.

Bugs
 
 
 
 
Dragging an item back onto the pedestal and removing it again will trigger the gate.
 
Leave the area and then return, the gates should be open.
 
 

Quest Stages