The demiplane known as Taer Umbra is an ancient outpost of the Qabalrin, belonging to the Shapers of Night. The demiplane has properties similar to Mabar, and it's energy can be used in Mabaran necromancy just as well as the actual energy of Mabar. As such, the Qabalrin established an outpost there to siphon this energy and use it in their rituals.
The Qabalrin are long dead, wiped out when the Heart of Siberys fell on the Ring of Storms in Xen'drik, but their outpost remained. It was discovered by Drow that fled into Khyber during the Scouring of Xen'drik, who would become the Umbragen. Taer Umbra is their home, and it is under siege by the forces of Belashyrra, the Lord of Eyes.
The demiplane is relatively small, by the standards of Khyber demiplanes, and most of it resides in a single large cavern, wherein lies the Fortress. This large cavern slopes down, with ancient structures built on terraces. The lower terraces serve as pools to gather the darkness, which settles there with enough density to resist dispersion due to light. The Umbragen use these pools to gather shadow to themselves, for use with their unique magic.
The upper terraces hold the actual Taer Umbra (Tower of Shadows), a fortress of dark stone built by the Qabalrin in ages past. The inner layers of the fortress contain libraries of necromantic lore, and the enshrined tomb of Shyntezca. The outer layers are dwellings, where the Umbragen reside, training areas and storage. Due to the war with Belashyrra, the terraces are regularly patrolled by the Umbragen, in addition to makeshift fortifications at the demiplane entrance that the beholderkin assault them from.