The Inner Sanctum of the Mask is the second chamber of the Altar Room.
Background[]
It is the holy sanctuary, the sanctum sanctorum which houses the Mask of Eternity in the Realm of the Sun. It has the Altar on which the Mask resides. The altar is positioned on the platform which can rise to the top of the Temple of the Sun.
Behind the scenes[]
The Inner Sanctum seems to be inspired by the concept of the Most Holy Place in the bibical temple, which housed the Ark of the Covenant.