“Its A Secret” Achievement Location
This achievement is unlocked by entering a certain room in Chapter 3.
So, to find this room, you want to go to Chapter 3: The Fallen Kingdom. This will spawn you in the Ishtar Chamber, and you will want to proceed through the stage until you enter the 2nd large open room with Ifrits (The Ishtar Temple).
Pass the first glowing purple spot, and you will see that to your left is a twisting tree branch that leads to an isolated platform with another glowing purple spot.
If you climb to it, and look out the back towards the ceiling, you will be able to see this building:
The glowing light is a hidden room within this structure. From the glowing purple spot, leave your body and possess an Ifrit. Use the Ifrit to fly up into the structure and enter the room. This will unlock the achievement, and reward you with quite a sight!