Prison of the Mad God

The Prison of the Mad God was the divine realm of Diinkarazan, the lost demigod of the derro,   after being driven insane and magically imprisoned there by the illithid deity Ilsensine. Nothing less than the power of a greater deity could free Diinkarazan from this realm.

Cosmography
In the Great Wheel cosmology, this realm was located in the Abyss,  in its 586 layer. Due to the metaphysical nature of the realm it was consistently balanced in this Lower Plane, but always on the verge of potentially slipping into Carceri or Pandemonium, the Lower Planes of imprisonment and madness respectively.

Description
In the center of this realm stood a single rocky throne, upon which Diinkarazan was bound. The throne was surrounded by a great swirling vortex storm of air and gas, with rings of whirling rocks flying about the throne. Beyond the throne itself, the size of the realm was fluctuating every minute, much like an area affected by the distance distortion spell.

Inhabitants
The sole inhabitant of this realm was Diinkarazan himself, who was tormented by illusions of his greatest fears. These included such things as drowning in lava or water, terrible monsters, and his imprisoner Ilsensine. The long imprisonment the deity had suffered left him blinded by revenge and feeling betrayed by his own people, thus he was liable to kill visitors to his realm.

Rumors & Legends
Legend had it that Diinkarazan was able to break free of this prison once every fifty years, during which time he would send an avatar out to the Prime Material plane to wreak havoc upon derro communities.