The Inner Sanctum is a room in Might and Magic Book One: The Secret of the Inner Sanctum. This is where the party encounters the data keeper, who gives them permission to use the Gates to Another World.
The party first learns of the Inner Sanctum when they escape from the Soul Maze, and Orango Seventeen tells them that "You are now Rank 1 and eligible for transfer. Find Inner Sanctum for new assignment." When they free King Alamar, he also tells them to find it.
The Inner Sanctum is located on the Astral Plane. To open the door, the party will need to find the key card and visit all five astral projectors. They enter the room to find a metallic room "dimly lit by a pulsating white glow". A metallic panel slides away, revealing a "mysterious man in a white coat" - the data keeper.
If the party has defeated Sheltem before visiting him, he tells them that "this is a rare occasion, for only a privileged few are given the opportunity for transfer to another V.A.R.N.", and allows them to use the Gates to Another World.