-Talk to Frea
-Find the source of Miraak's power
-Read the Black Book
-Speak to Frea
At the Temple of Miraak you will come upon many enthralled workers building a shrine. Although several of them are Reavers, none will harm you. Speak to the only sane Nord Female named Frea, who is attempting to snap her people out of their trance. She comes from a tribe of Nords known as the Skaal, who are very shamanistic compared to the Nords of Skyrim. The only reason she still retains her own mind is through a necklace she carved herself. Only a few of her tribe still remain, thanks to a temporary ward set up by her father, the village Shaman. Agree to follow her into the temple where you will most likely be set upon by cultists. Defeat them and enter the temple.
Once inside you'll go through several rooms full of loot, Frea will wait while you scavenge anything of value. Continue to a large chamber with a pit in the middle and several skeletons hanging from cages. Frea will mention the likelihood of treasure on top of a seemingly inaccessible platform, so look for a way to climb one of the slanted pillars to find a way up to a locked chest. Be wary of the pit however, as several cultists will emerge from the stairs leading out of the pit to attack you.
Head onward through the crypts and be wary of the Draugr who like to spring from their resting places to attack you. Several traps will also be hidden, so be mindful of where you step, unless you plan to lure the Draugr into them. Eventually you will come to a long corridor filled with swinging blades. Frea makes the smart decision to send you on ahead. Unless you fancy yourself an artful dodger, you may want to consider using the "Become Ethereal" Dragon Shout to bypass the traps and deactivate them for Frea. Not long after this room you'll happen upon a large chamber with 3 bridges and a very high level cultist. Mind the floor traps and take him and his cronies out. Once defeated you will be able to enter Miraak's Sanctum.
Be sure to follow Frea whenever she leads onward as she might show you some rooms full of treasure. After clearing a large open room, head down the stairs to find a Dragon Word Wall. Obtain the word to learn Strength of the Dragon Aspect Shout. Several Draugr will leap out from their coffins right away. Kill the leader to obtain an iron key that opens a door which can be found behind one of the coffins. Later through the tunnels you will come to an area with some creepy statues and two spiraling staircases. Battle onward until you come to an area that slopes upward and is home to several dragon skeletons. Many Draugr and Skeletons roam this area, and the slopes are laden with falling rock traps, which can either be avoided or used to damage the incoming enemies. At the top of the room lies an ominous statue of Hermaeus Mora, a bit of foreshadowing as to what is to come.
You will then come upon a room with a large Black Book on a pedestal. Reading it will trigger your transportation to the Daedric realm of Apocrypha. It is there that you will witness Miraak himself, who claims you know nothing of the true power of a Dragonborn. To demonstrate, he uses the three words of the Dragon Aspect Shout, covering himself in magic armor. He then orders his minions to cast you out of Apocrypha, as he makes his exit on the back of a dragon. Once back in the Temple, speak with Frea who will suggest you head to her village. Follow her out of the temple and towards the Skaal Village, past the Wind Stone where many of her people toil on in a trance. Speak with the Skaal Shaman to complete this quest and start the quest, The Fate of the Skaal.