Mabar, the Endless Night is a blackened realm where the faintest spark of light is quickly devoured by the unending darkness. It is a plane suffused with negative energy and the antithesis of Irian.
Mabar is a realm where no light shines. It is a world that is home to demons and the creatures that go bump in the night.
On Eberron, one can be brought back after death as either an undead or as a deathless. Those that are undead are said to have been infused with the essence of Mabar, while deathless have been infused with the powers of Irian. There are those that worship the deathless of the Undying Court who wish to remove all undead from the world. These Deathguards will hunt down Mabar manifest zones, trying to sever their connection with the realm.
On the flip side, priests of the Blood of Vol will also seek out manifest zones of Mabar, to build shrines and allow access to more powerful spells. In addition, a sect of druids called the Nightwalkers also worship Mabar, and seek out manifest zones, especially the Gloaming.
Coterminous: The nights grow blacker and more dangerous when Mabar becomes coterminous with Eberron. Travel to Mabar becomes as easy as stepping into an area where there is no light, and inhabitants of the plane slip out from the shadows and into the world.
Remote: When Mabar is remote, the darkness seems to diminish from Eberron.