The Plane of Shadows is a non materiel plane that touches the Prime Material in all places and touches other Planes as well. It is sometime called the Ethereal Plane.
In form the plane is a dark echo of the Prime Material. It is literally dark, the place is in perpetual twilight. Buildings are echoed, but only if old. Some might be present that no longer exist, but have a heavy psychic imprint. In short a phantasmagorical landscape that is difficult to navigate.
One will see people here. They are the shades of the dead on their way to the afterlife. Sometimes fay creatures than can travel the planes at will. There are also incorporal predators that wander the plane.
Unless you have very good reason this is a place to avoid.