Talk to Frea
Have a chat with Frea and she'll join you on your quest to beat the life out of Miraak, but first you'll both need to head down into the Temple and investigate what's going on with all of the possessed people.
Find the source of Miraak's power
Frea will push on down the spiral stairs leading into the Temple, slap the cultists around until they're all dead (loot them for any valuables that interest you) and then open the doors leading into the temple itself.
Loot the rooms in the initial corridor for a multitude of potions and coins, but pay extra attention to the far left room where there's a chest containing some very handy [ Dwarven Boots ] (along with some useful potions on the shelf opposite).
Be extra careful from this point on and keep an eye on the floor for stones with spirals on them as these are trap triggers (for such nasty things like poison darts).
Once you reach the large room with the cages hanging over a void, be very careful as there's an ambush lined up with a rather powerful Draugr Deathlord who's capable of removing your main weapon away from you with magic!
The best way to deal with this enemy (and the other Draugr's) is to let Frea do most of the grunt work (seeing as she's invincible). You can loot his corpse for a unique enchanted weapon [ Ancient Nord Battleaxe of Ice ] .
Proceed down the stairwell in the middle of the room and Frea will destroy the cultists that get in your way. In the next tunnel, activate the handle then watch out for all the Restless Draugr's that show up to the party. When you reach the split in the path, take the right side, loot the chest then be very careful when approaching the double doors as there's a trap switch in middle of the ground (that sets off very painful flames).
In the next room you'll find a long corridor with loads of swinging blades, make your life easier by using the 'Whirlwind Spirit' shout and dashing through all of them to the switch at the end that deactivates the trap.
In the next room there's a handle on the right that opens up a path to a secret room with a few Soul Gems, a Black Soul Gem and other, minor, items. The next room has two Draugr's, trap switches and a Cultist Adept that you need to fight, so be careful!
Take out the Cultist Adept first as he's in the middle, then focus your attention on the two Draugr's on either side. Once you're finished, open up the door leading to 'Temple of Miraak Sanctum'. Watch out for the trap floor tiles where the Draugr's appear, then when you walk past the metal bars hag a right and drop down into the hole.
You'll reappear in a hidden area where you'll find large Soul Gems, coin bags, the spell books [ Spell Tome: Conjure Flame Atronach ] and [ Spell Tome: Magelight ] and various potions. Nice haul!
Proceed onwards, kill the two Cultist Adepts that show up, there's a unique enchanted weapon [ Steel Sword of Ice ] on the lower floor where the Cultists appear and then hang a left until you reach some stairs leading down. Save here!
The next room offers up a new word for the Power Shout, but in doing so, you'll trigger off a mini-boss fight with a Draugr known as the Gatekeeper (who can take a decent beating). Your best tactic is to get behind it as it attacks Frea and beat it down whilst it's distracted. Otherwise it can drop you in very few hits.
Grab the [ Temple of Miraak Key ] from its cold corpse and use it to open the door that can be found in the coffin the Gatekeeper came from. The next area with the numerous bookshelves has loads of ingredients and potions in it, and there's handily an Alchemy Lab in a corner for mixing them all together. Bonus!
Now activate the nearby handle, head back into the main room and go through the newly opened secret passageway. Activate the handle in the middle of the room and proceed down the spiral stairs. At the very end of the next few, enemy free, corridors there's another handle to pull.
Run down the newly revealed stairwell and once at the bottom save your game! You're about to fight numerous Draugr's and a very powerful Draugr Death Overlord.
Pick off the Draugr's as you slowly make your way up the steps (be extra careful of the rock traps located up and to the top on both sides). When you reach the top, the Death Overlord will show up, so lure it into attacking Frea and then hit him hard from behind with your most powerful attacks.
He can take Frea down to the point where she has to recover for aroud 20 seconds, so if you see her drop to her knees back off and find cover! The Overlord has very powerful shouts that can kill you in seconds.
When it's finally dead, you can open the chest at the top of the room for some very useful Heavy Armor [ Dwarven Armor ] . Pull the level at the back of the room then run until you reach the room with the book in it.
Read the Black Book
Pick up the book and then open it up from your inventory to trigger a cut-scene. Once it's finished talk to Frea.
Talk to Frea
Talking to Frea will complete this questline and begin the next one in the main storyline.
DLC: Dragonborn - The Temple of Miraak
Complete "The Temple of Miraak"