Mabar

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.
In the Eldeen Reaches, a section of the Towering Wood called The Gloaming is a Manifest Zone to Maban, resulting in disfigured trees and Horrid Animals. In the Faded Forest of Breland, a large mystical arch has been crafted. It opens a portal to Mabar every tenth night and turns the entire area into a manifest zone to Mabar during the month of Sypheros. The Shanjueed Jungle on the continent of Sarlona is home to a manifest zone to Mabar. After the Day of Mourning, Valin Field in Cyre became a manifest zone with both Mabar and Shavarath.
The nights grow blacker and more dangerous when Mabar becomes coterminous with Eberron. This happens on the three nights of the new moon closest to the winter solstice once every five years. When Mabar is remote, the darkness on Eberron seems less deep and the nights less fearsome. These periods occur every five years on the five nights around the full moon nearest the summer solstice.