The first step is to head to The Slumbermead section of Nabreus Deadlands. This area is located south of the orange teleport save crystal.
From there, go to the western most bridge of the area and face north. If you tilt your camera slightly to the left and up with the right analog stick of your DualShock 4 controller, you will find an underpass between two big rocks.
Head under it and you will get to an unmarked area of the map. Take an immediate left and you should see the blue line to head to the next hidden zone.
The Fog Mutters
The next area is fairly simple. Just follow it and to the next zone and the path will lead you straight to Overlooking Eternity.
After doing what you have to do in Overlooking Eternity, you can exit the area. This can be a little tricky if you did not pay attention to how you got into The Fog Mutters. Simply head east in the Fog Mutters Area. If you look carefully, you will see a hidden path submerged underneath water. This will lead back to The Slumbermead.