Inner Sanctum

It's a sublocation of the The Forgotten Vale.

Exterior
The Inner Sanctum is a large, Elven inspired building that appears to be made out of a white marble or similar stone, with large ornate gold windows. The Sanctum is located just past the Wayshrine of Radiance.

To gain access to the interior, the Dragonborn must have visited all of the Wayshrines and filled the Initiate's Ewer during Touching the Sky. Upon doing so, the Ewer is then emptied into a basin near the door which then flows towards the main door and unlocks the seal.

Inner Sanctum
The interior is a large chamber with a large number of frozen Falmer and Chaurus inside. In the center of the main room is a Shrine to Auri-El.

On the left wall, there is an altar where the Initiate's Ewer can be placed and that grants access to a hidden room with a Frost Giant inside. This giant holds the Ruby Paragon that may be looted from it's corpse if killed. There is also a second altar on the far right of the hidden room that, when activated with the Initiate's Ewer, reveals more hidden loot.

If the Sapphire Paragon is used at the Paragon Platform, it will grant access to an otherwise inaccessible portion of the Inner Sanctum that contains some Elven Arrows and other minor loot.

The rear of the sanctum connects to a glacial tunnel that leads to Auriel's Chapel.

Auriel's Chapel
The chapel is mostly in ruins and has been overwhelmed by the elements. The initial collapsed hallway opens up into a small cavern housing several frozen Falmer and Arch-Curate Vyrthur seated on a throne. Behind the throne a large balcony is located.

Inner Sanctum Balcony
The grand balcony overlooks the Forgotten Vale. In the center is a Wayshrine of Auri-El that can be used to travel to other parts of the valley.

Notable items
There isn't much notable loot in this area apart from the Ruby Paragon. However, the frozen Falmer are often carrying magical staves of some kind, or various other useful items such as potions, jewelry, particularly one carrying a semi-rare Waterbreathing enchanted crown, or spell tomes.

Appearances

 * The Elder Scrolls V: Dawnguard

Bugs

 * Sometimes, for some unknown reason, you will not be able to enter the Inner Sanctum and the game will freeze upon the start of the loading screen. To fix this, leave the area for a while and return to try again.
 * Sometimes when entering the Inner Sactum, the skeletal corpse of 2012-08-15_00002.jpg dragon Naaslaarum will appear with you when you first zone and drift towards the opening you came through, where he will promptly get stuck and sit there twiching.
 * Supposedly this will only have a chance to occur if the Dragonborn already slew Naaslaarum and Voslaarum in the Forgotten Vale.