The Umbral Plane

The Umbral Plane is the plane of grays and black. Its also known as the inverted world or the plane of darkness. The plane is filled with things most say shouldn't exist. It is directly connected to death. Spellcasters who deal with shadows and death usually use this plane for their magic as it is safer to use than the plane of oblivion which is the source of negative energy and where undead and daemons originate. Due to the Umbral planes connection to the Oblivion plane undead and daemons sometimes spill over or suddenly appear within the Umbral plane. Some souls also escape from the Plane of Passage and become undead.
There is many dangers on this plane and it is not recommended to travel about it in case you run into the Veiled Gods. Which are a collection of beings that have existed seemingly since the beginning of the Umbral Plane perhaps even before. These beings enjy causing misfortune and death upon the denizens of The Mortal Plane.