The Dim Wastes
Formerly a priest of Fen named Arugal Xazat. Deity of the Shadow plane. Overpowered and expelled all other gods, which can no longer enter it. Essentially, everything in the shadow plane is an extension of his consciousness.
There are a few powerful and smart entities that already existed on that plane and remain. Some of these are Shade Lords. Mythic heroes of the Shadow plane. Xazat’s generals, with power granted by Xazat to help keep them in line. They help manage underlings and maintain the plane.
The Shade Lords were present when Ghadir sacrificed two final members of the House Tulesti in order to create the Twin Skulls of Tulesti, and the Twin Skulls can only be destroyed by defeating those who were present (three of them).
There are many material planes, but only one shadow plane, as with various elemental planes etc. As Xazat learns of material planes, when he can, he absorbs the material planes into the shadow plane. He gains power from these absorbed planes, and expects an eventual war of the planes, for which this is preparation.
The War of Passage occurred when the elves became too powerful and started siphoning power from the shadow plane, and Zezat got unhappy.