Head to the Northern side of this temple. Take the ramp downward. There are a pair of Cultists waiting just down the ramp.
Go down the ramp and into the first corridor. There are 2 rooms off to the sides, one to the left and one to the right. You will find some healing potions and a chest with some minor loot in them.
Go to the North and then turn to the West with the corridor. This leads to another room. In the entrance is a pressure plate. Stepping on it will trigger a dart trap that looses a barrage on you. You will have another pair of Cultists and a Draugr in the coffins to the left and right. Clear them out and then do through the door to the West.
Follow this corridor into the next room. Head around to the Western side of the room and turn to the North to continue forward through the doorway there.
In the next room be careful, there are 2 pressure plates. One is just inside to the left and the other is opposite where you enter on the same side. They can trigger battering rams on both sides. Go forward past the first pressure plate then go over to the left between the columns.
Continue to the West down the stairs. There is a pull chain on the right-hand side of the door. Interact with it to continue forward.
In this next chamber just wait for Frea to come in and talk. She will comment on what could have happened in the cages above you. Then she will say she sees something.
Follow her to the North and climb up the stairs. Turn to the right and climb up onto the angled column there. Go up it to about the mid point and look to the right. You should be able to jump over to the top ledge.
Collect the loot inside the chest and then turn to the South. Drop off the ledge you just climbed up to and then start dropping into the pit in the middle of the room. Be wary because a pair of Draugr will emerge from their coffins in the Northwestern part of the room.
At the bottom of the pit, turn to the North and head forward slowly. There are more Cultists waiting for you in that corridor. Take the stairs downward after you kill them.
Turn to the East with the corridor. There is a handle in the pedestal that you can interact with to open the closed door. Do so and continue forward.
This next room has more traps, but they are fairly easy to avoid. Start by going to the left and then following the carpet to its end. Stop there and look right. There is a Restless Draugr that can appear from the wall there. Follow the corridor to the North but be warned there are more Draugr, and likely a [Skyrim: Draugr Deathlord|Draugr Deathlord]].
Continue forward to the North. be careful of the pair of pressure plates ini the middle of the corridor. Keep to either the left or right-hand side to avoid triggering the battering rams that are above you.
Pause the split in the corridor. There are a few Draugr to be found on both sides of this corridor. Clear them out the go up the slope to the North and follow it forward.
Go with the corridor over to the West and head for the open double doors. Watch carefully with the pressure plate that is in the narrowed portion of the corridor just before the doors. Jump over it to avoid a flame trap that also closes the doors leading into the next corridor.
Inside the long corridor, get to the end by moving carefully between the swinging blades. Whirlwind Sprint can be very helpful to get through quickly. At the far end, go to the right-hand side of the door. You will find a lever there. Pull it to move forward.
Go down the stairs and open the door to the South. This leads to another room. Go to the far Southern wall and turn to the left. There is a Dragur to the right that will emerge from its sarcophagus.
Follow this path to close to the end of it. Look to the left and you will find another handle to pull. Do so to open the door blocking your way forward.
Head on through the door to the West of the handle you just used. Wait for the bridge to fall while you are in front of it and then go down it.
Turn to the left, being mindful of the pressure plate at the base of the bridge. Step on it when you are ready for the next fight.
Stepping on the plate will drop the three bridges on the opposite side of the room. There is a high level Cultist and a pair of Draugr Deathlords. Killthem and head through the door on the far side of the room. This will bring you into the Temple of Miraak Sanctum.
Go forward through the caged walkway. Toward the end of it, up the slope from its base, are a few Skeletons. Take them out and continue on forward.
Go to the end of the corridor. Follow it to the South. Be careful of the traps that trigger at the base of the stairs. These battering rams are found throughout the corridor you are in. You will also need to deal with a pair of Draugr.
Follow the pathway forward into the caged portion. Go to the East from there and follow it to the next room. There are more [Skyrim: Draugr|Draugr]] that will come out of the Southern coffins.
Go to the Northern side of the room and go to the West through the corridor there. Go on down the stairs there.
At the base of the stairs you will emerge into a room with a dragon skeleton above you. Look to the South to find a Word Wall. This has the word Strength for Dragon Aspect.
Turn around to the North and deal with the 4 Draugr that are coming out of the coffins.
Defeat them and the Gatekeeper will emerge from the final sarcophagus. Work with your Companion and Frea to defeat this powerful undead.
Go the sarcophagus that the Gatekeeper came out of. Interact with the back of it to open the door inside. This brings you to the next room. Go to the Western side of the next room and open the door with the key again.
This leads you into a dining area. Go to the Northern side and over to the left. Go first into the doorway and then down the first corridor on the right. You can go into the room beyond for more conversation with Frea.
About the far end of the corridor between the two rooms.
You will find a handle that you can interact with to reveal a hidden passage in the dining area.
Go back into the Dining area you entered and go to the new opening to the right between the bookcases.
Follow the corridor through to the next room. You will find a room with a number of books.
Go into the room in the back right. Against the Western wall you will find a statuary above a handle. Go and interact with the handle to open the way forward. This will open the grate in the floor just behind you.
Drop down into the opening and then go forward to the East.
Go into the next chamber and drop down again to the first doorway to the West.
Go to the southern end of this corridor. This looks like a dead end, but interact with the handle to the left of the dead end. This will reveal the continuation of this corridor.
Head down the stairs behind the newly revealed passage. Go through the wooden door at the end of the passage. This leads you to a massive cavern with numerous dragon skeletons.
Make your way up the stairs on either side. Be careful and move up the stairs slowly. There are a number of traps here that will drop boulders on you. You will need to deal with Skeletons and some Draugr.
Get to the top of the stairs and you will find a Draugr Death Overlord. Defeat the Draugr and then go to the South behind the statue.
Follow the passage there to its end. You will find a pedestal with a book on it. Wait for Frea to come into the room.