Siege on the Dragon Cult

Overview
Talking to Captain Valmir at the entrance of Forelhost starts the quest. He asks you to retrieve a mask for him out of the caves. The mask he is referring to is a Dragon Priest Mask known as Rahgot. He also, as an optional task, asks you to find Skorm Snow-Strider's Journal.

Objectives

 * Obtain Rahgot's Mask
 * (Optional) Find Skorm Snow-Strider's journal
 * ​Return to Valmir
 * Kill imposter Valmir

Quick Walkthrough

 * Speak to Captain Valmir to begin the quest.
 * Enter the ruins of the stronghold 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, and move on to the Refectory.
 * Travel through the Refectory, find the Glass Claw, and locate the dragon claw door.
 * Use the claw to enter and defeat Rahgot.
 * Return to Valmir with the mask.

Captain Valmir
If you pickpocket the Captain (if you get caught & he attacks you, just kill him & carry on with the quest as normal), read "Valmir's Orders" (or you will get these orders at the end of the quest), you will find out that Captain Valmir is an impostor. The orders read:

"You will proceed to the ruins of Forelhost to retrieve the Mask from the Dragon Cult there. If you are discovered, impersonate an officer. It is unlikely that anyone from Skyrim will be clever enough to see through the disguise. Once you have obtained the Mask, bring it to Labyrinthian."

Undead
Once inside Forelhost, you will encounter many undead dragon cult Draugr.

The undead at the entrance aren't that difficult, but as you get deeper into the dungeon, you will encounter Draugr and Draugr Scourge (the ones that can actually do dragon shouts, some of them even summon Frost Atronachs) and Draugr Deathlord. These are pretty hard, and are usually accompanied by a few other enemies. In one area there are 2 Draugr Deathlord's at a time along with weaker allies. Be aware that in all likelihood these enemies are leveled. If one enters Forelhost having a level of at least 59, instead of Draugrs, you face all Dragon Cultists.

Traps
There are a lot of traps in this dungeon. There are floor plates that trigger various traps. Some of the chests are trapped. There are swinging axes in slim hallways. There are mage runes in random spots that blast you if you get too close. There are trigger plates that lift when you take an item that is sitting on them. There are soul gems that are rigged to shoot lightning or frost at you (high damage) if you get in line of sight with it (these are wicked and will drop you fast). There is even a room that I found where the floor raises up as you step on it and impales you with spikes in the ceiling. Be very cautious! There is also a trap you can use to your benefit, it is one that swings towards you when you step on a pressure plate, and it does have spikes on it.

Misc Materials
There are quite a bit of misc items and materials in this dungeon. In the room with the dead spider, there is one gold ore vein, one quicksilver vein, and a lot of glowing mushrooms. There are a lot of alchemy materials in a few rooms since some of the undead are/were alchemists. There are a few alchemy tables. There is also an enchantment table right before entering the final room. There are some rooms with smithing materials and forge/grindstone/work bench (no smelter that I saw). There are a LOT of other random things in here, if you want to load up, make sure you are really light before entering (or do multiple trips back to a merchant).

Journal
As you progress through the dungeon, you will eventually find Skorm Snow-Strider's Journal. It is just sitting on a desk. Don't bother bringing it back to the Captain right away. He just complains that you haven't found the mask yet. Reading the journal is pretty interesting and details Skorm's adventures leading him to the dungeon and discovering the Dragon Cult. He mentions something about a key and that it should be found in the dungeon.

Dragon Claw key
The Dragon Claw Key for this dungeon is the Glass Claw. It can be found in the room where you are attacked by a bunch of Draugr and a soul gem that is shooting lightning at you. The claw is found on one of those waist high, dark gray pillars. As you pick it up off the pillar, the little platform that it is laying on will rise. This will close a gate near you, blocking your passage. You have to find something else to place on the pillar to reopen the gates.

This claw will open the Nordic puzzle door which leads to the final battle. Pattern from top to bottom is Fox, Owl, Snake.

End battle
Just past the Nordic puzzle door, you will enter a fairly large room where you will do battle with Rahgot and a few other Draugr. He, like all named dragon priests, uses a staff, this particular one is a firewall spell.

He will also have a key on him that will let you open the door in his lair.

Finishing the Quest
Once you are finished searching the room for random loot, head out the big doors (with the key). Once you are outside, you will be on a ledge just above where Captain Valmir is. DO NOT fall/jump off the ledge yet. Head to your right and you will see a Dragon Shout wall where you will learn the Storm Call shout. After that, jump down and talk to Captain Valmir. Note that if you DO jump down before getting the shout, you can get back up using the structure around the entrance to Forelhost. It takes a bit of jumping, sliding, and using slight out-croppings on the wall to get up, but it's better than running through the dungeon again.

He will be trying to lure another soldier into the dungeon to retrieve the mask. As soon as he sees you, he stumbles on his words trying to make up an excuse, but then realizes that he has been caught and attacks you. The soldier attacks you as well, but they both die very easily. As soon as the Captain is dead, the quest is complete.

Video Walkthrough
A detailed video walkthrough can be found here.

Notable Loot

 * Rahgot Dragon Priest mask
 * One word of Storm Call shout

Bugs

 * In the hallways with the swinging axes, companions can get hit with the axe and get pushed into the wall and disappear.
 * Sometimes the puzzle door won't open, regardless of whether your puzzle combination is Fox, Owl, Snake. Preventing you from getting the mask and the Storm Call word.
 * Sometimes Captain Valmir won't talk to you to start the quest. You can pick his pocket for the key and go inside. Once the mask is retrieved the quest journal will update, instructing you to return it to him. He will still refuse to talk to you, but after he blunders his excuse to you then he and the soldier will attack. This does not appear to complete the quest, leaving it broken.
 * During quest initiation, Captain Valmir may also stop talking to you if a dragon starts attacking. Attempting to speak to him afterwards will not start the quest.
 * It is possible for the pieces of the door puzzle to stop moving, making it impossible to get past the door.
 * Occasionally the pedestal for the Glass Claw will not allow anything to be placed on it, not allowing the bars to go down and trapping you in the room.
 * Captain Valmir may greet you to start the quest dressed in Imperial Gear and relate General Tullius to the issue, but after the dungeon is completed be dressed in Stormcloak Officer gear and be talking to a Stormcloak soldier.
 * To the comment above; thats not a bug. Its done on purpose to show that he is an imposter.