Demoncyst

Demoncysts were pockets of the Hidden Layer, the 248 layer of the Abyss, that had become linked to the Prime Material plane and scattered across the Demonlands.

Description
Demoncysts were enclosed spaces containing fragments of the Hidden Layer, a barren and rocky plane of violent lightning storms. The cysts were typically 100 feet (30.5 meters) or more in diameter. The Abyssal environment inside them did not interact with the surrounding terrestrial environment (and vice versa), although creatures could freely pass into and out of them. Thus, each demoncyst appeared as a fully self-contained cavern or bubble of the Hidden Layer's weather and topography.

Geography
Demoncysts were scattered across the Demonlands of northeast Faerûn. They were mostly concentrated beneath Narfell, the Great Dale, and Thaymount, but extended as far west as Vaasa. They were often buried underground, and could be found beneath Damara, Impiltur, Thesk, and the tharch of Priador in Thay.

History
The demoncysts first appeared circa, when Eltab was summoned by Narfell to fight in the Great Conflagration against Raumathar. Somehow, this summons also bound pieces of his Abyssal realm, the Hidden Layer, to the Material Plane, and these pieces became scattered across northeast Faerûn. The locus of this binding of the two planes was a calling circle called the Adamantine Binding inside Eltab's palace, which was located inside the largest demoncyst, known as the Hall of the Hidden Throne, in the depths of the Citadel of Conjurers.

In the, the Mulhorandi deity Anhur imprisoned Eltab within a demoncyst beneath Thaymount.

In the 600s DR, settlers in the Great Dale uncovered many demoncysts, and inadvertently released many of the inhabitants.

In the, the Zulkir of Conjuration Jorgmacdon ruptured a demoncyst when he temporarily released Eltab to fight for the forces of Thay, causing waters from the Styx to flow out of the cyst and create the River Eltar.

Eltab was freed from his imprisonment in the. Although free, Eltab was unable to return to the Abyss due to the Adamandine Binding, and so returned to the demoncyst instead. In the, the demon lord was driven out and resettled in the demoncyst of the Hall of the Hidden Throne.

Inhabitants
Some demoncysts contained demons, half-fiends, tieflings, or other servants of Eltab, while others were empty save for whatever entered from the Realms.