New main objective: Obtain Rahgot's mask
New optional objective: Find Skorm Snow-Strider's journal
Notice! This quest is connected with the Dragon Priests Masks, so it's best to complete it while you're collecting one of the eight masks.
Open the world map and head to the south-east part of Skyrim, to Forelhost (screen above). if you haven't discovered it yet, it's best to begin your journey at the nearby Riften. After reaching the destination, you should see a camp set by Captain Valmir, with whom you should talk. He will tell you that he's a member of the Imperial Legion and ask you to retrieve the mask of the Dragon Priest who supposedly can be found here. He will also mention Skorm Snow-Strider's journal, which would be good to find as well.
Enter Forelhost Stronghold and be ready for a long and hard journey. At first you will have to mainly fight with ghosts of Cultists, who don't require any special strategy. Look out for the numerous and often dangerous traps which you should always try to avoid. Skorm Snow-Strider's journal can be found in the north-east part of the location, guarded by a couple ghosts (screen above). I'd of course recommend reading it beforehand.
Keep heading further, reaching the passage to Forelhost Crypt (screen above), where you will come across Draugrs for the first time. I'd suggest being very careful, as you will mainly come across their stronger kinds, like Draugr Wights and Draugr Scourges. Adapt your strategy depending on whether you're fighting a warrior, archer or mage.
Use the door leading to the crypt and explore the new location. Look out for more advanced traps, including swinging blades. Reaching the passage to Forelhost Refectory shouldn't take you too much time. There you will come across Draugrs more often than usually. Eventually you should reach a library in the northern part of the Refectory and take the Glass Claw from the pedestal (screen above). By doing so you will unlock the south-east passage.
Soon enough you should reach a locker door, where you will need to solve a puzzle resembling those which you have probably come across many times before. Watch the Claw from the inventory level and set the symbols in the same order. The upper ring needs to show a wolf, the middle an owl and the lower a snake. The full solution can be seen in the screen above. Use the claw on the door to unlock a passage to the chamber where the Dragon Priest dwells.
Start fighting Rahgot. Try to avoid his spells and don't let the Draugrs that are accompanying him to surround you. You should take care of them only after killing the Priest. Afterwards search his body to take the Forelhost Balcony Key and most of all Rahgot Mask (screen above)
New main objective: Return to Valmir
New main objective: Kill the imposter Valmir
Before leaving this area, be sure to check the treasure chest. Only afterwards use the newly gained key to open the door leading to Battlements and therefore to the surface. Head back to the entrance to Forelhost Stronghold and note that Valmir is trying to convince yet another character. Be ready, as they will soon both attack you (screen above). Start off by killing the Stormcloak Soldier and take care of Valmir afterwards, at best keeping him at distance. After the battle, you can search the imposter's body to find Valmir's orders and also look into the chest with his belongings.