After entering the Temple of Miraak Sanctum, go down the stairs and through a narrow passage with skeletons hanging from both sides. Frea remarks that she does not know what it is Miraak learned that gave him reason to turn on his masters, but his path seems to have been a cruel one. She hopes you will find some answers to what happened so long ago.
At the other end of the passage prepare to fight some undead and other enemies. But watch out for additional traps, in particular battering ram traps. Arrive at a small circular caged chamber, and turn left. There is a small room here with spellbooks and other stuff so be sure to loot what you need. Soon you arrive at a long corridor with stairs leading down. This leads to a room with a big dragon statue, and to the left there is a wall with inscriptions. Approach the inscriptions to learn:
Word of Power: Dragon Aspect
You will be learn one of the following words of the Dragon Aspect Shout:
Strength, Dragon Aspect
Armor, Dragon Aspect
Wyrm, Dragon Aspect
Depending on how many words you have learned so far. For a full strategy guide on how to get all three words of the Dragon Aspect Shout, see:Skyrim Dragonborn: Dragon Aspect Shout All Three Words
As soon as you finish learning the word of power, Draugr will come out of their coffins and attack you, as well as a unique enemy called the Gatekeeper. Kill the Draugr and the Gatekeeper, and loot the Temple of Miraak Key from the Gatekeeper. There is also a chest in this room.
The key you looted from the Gate Keeper opens a door in the same room. The locked door can be tricky to find, but look in the area from which the Gatekeeper came out, under the dragon skeleton statue, to the left of a chest. Here is what the door looks like:
Go through that door to advance to the next area in the temple.