Mabar

Mabar, the Endless Night, is a plane suffused with negative energy, and is the oppositional counterpart to Irian. Whereas Irian is a land of light, Mabar is a land of darkness that doesn't permit any light to exist. On the surface it would seem to be the Plane of Darkness, but the darkness is merely a side effect of the plane's true nature; All day's eventually end, and the light is eventually snuffed out and replaced with the dark. Mabar represents entropy, the eventual end of all things, including light itself. Whereas Dolurrh is the plane of the dead, Mabar is a plane of death.
Mabar is the source of negative energy in the planes, and it is this energy that animates most undead. Manifest zones to Mabar have an entropic effect on the environment, draining the life from all things. Mortal creatures that find themselves exposed to Mabar quickly die unless they have protection against it, and Undead that reside there are empowered by it. Just as Irian empowers the light, Mabar snuffs it out, and spells and effects that generate light or heal wounds fail.
Most of the planes are relatively structured, even Dal Quor is relatively stable despite being shaped by the dreams of mortals. Mabar and Irian break this trend. Mabar pulls fragments of other planes into itself and assimilates them over time, as they are drained of light and life. Initially for those within the affected region, night falls and never ends. Slowly but inevitably, the creatures die and become undead, and any immortal spirits present are corrupted into twisted mockeries of their former selves. Once the region has become fully corrupted and suffused with the energy of Mabar, it remains as a permanent memorial within the plane.
The following information is not known to the people of Eberron, and so has been marked as a spoiler.
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This effect is not limited to the planes, Mabar can rarely consume fragments of the Material Plane as well. A grey mist falls upon the effected region, and anything caught within it is lost. This fog fades fairly quickly, leaving a barren landscape stripped of all structures and life, and so effective is Mabar's consumption that it also consumes any memories of the place. Mortals forget the location ever existed, losing any memories they may have had attached to the place or the people that were lost.

The inhabitants of Mabar all embody aspects of darkness, death, and decay. Most of the inhabitants are shadows, and much like every mortal has a spark in Irian, it is possible that every mortal has a shadow in Mabar. The immortals of the plane represent the planes concepts, whether they formed in Mabar originally or were corrupted after they were drawn into Mabar.
Mabar is the source of most undead, and the necromancy practiced by the Qabalrin and the Blood of Vol channels the energy of Mabar to create undead. The negative energy that suffuses the plane sustains the undead created by Mabaran necromancy, and that undead acts as a conduit to the plane. As well as the undead on the Material Plane that are sustained by Mabar, there are also a significant population of undead that reside within the plane itself, and it's believed by some that the souls of those who become undead are bound for Mabar when they are eventually destroyed, rather than Dolurrh.

Type

Dimensional plane