Veil

The Veil, sometimes called Limbo or Purgatory, is not one of Plane of Existence, but a pathway or a tunnel that lies on the border between the Material Plane, the Spirit Planes and the Underworld, and can only be accessed from one of its connected Planes. Some have theorised that the Veil is actually an extension of the Land of Shadow.

The Veil is a strange and ethereal world that exist on the border between the world of the living and the world of the dead. When a Mortal creature dies, their Soul passes into the Veil, where they are usually met by one of the Psychopomps, who are usually unable to pass through the Divine Gate, and instead use the Veil to traverse Planes in order to escort newly deceased Souls to their designated afterlife.

Outside of Psychopomps and Spirits, a majority of creatures can not interact with or travel into the Veil without the aid of Magic, and even then it is only temporary, with the Veil being incompatible with living creatures.

Spiritual Inhabitants

Besides Psychopomps and exploratory Mages, the Veil is almost exclusively inhabited by the Spirits of lost Souls. Often these Spirits have gotten lost on their way to the Underworld, or have escaped being ferried by one of the Psychopomps, and have been transformed into malevolent Ghosts, Wraiths and Spectres, clinging to the memory and trauma of their former lives.

Spirits trapped in the Veil can use it as a pathway to travel between the other connected Planes, commonly using it to cross over from the Land of Shadow into the Material Plane at night, to haunt the spaces they frequented in life. 

Distorted Reality

The Veil is said to resemble the Material Plane, but as though everything was covered in a veil of ghosty and distorted white.

Creatures within the Veil are invisible to those upon the Material Plane, and those on the Material Plane are invisible to those within the Veil. Creatures within the Veil are also said to be able to touch objects within the Material, but they are unable to move or interact with those objects.

While this seems to be the general rule, reports suggest that certain Spirits are capable of seeing through the Veil, and can even interact with, or move objects from within it. But this action seems to require great effort.