Demiplane of Imprisonment

The Demiplane of Imprisonment was an extremely obscure demiplane. Very little was known about it.

Geography
The demiplane resembled a crystalline cyst floating through the Ethereal Plane. The ether for around the demiplane continuously manifested dreamscapes for the nightmares that an intelligence within sent around the multiverse to its would-be worshipers; these were dominated by purple eyes, mouths, veined whirlpools, and dark humanoid figures. The visions continuously attempted to goad, tempt, and seduce travelers into touching the crystal, which would trap them in the demiplane.

History
Sometime after, the archwizard Madryoch was banished to this demiplane. The plane was rediscovered later by Netherese arcanists following up on the work of the archmage Shadow; a handful of them became imprisoned in it. Shadow himself used assassins he caught alive as exploration guinea pigs, making them touch its surface; he never managed to retrieve them afterwards.

Inhabitants
People who touched the crystal were absorbed into the demiplane. No one could leave on their own power; it took a spell like wish or miracle from someone outside the demiplane to escape. Those removed would be under the effects of an insanity, that could be healed in the same ways as the effects of the spell of the same name.

The archwizard Madryoch had been banished to this demiplane. Madryoch's essence later escaped the imprisonment, instead being trapped in the Plane of Shadow.

Legends
The Demiplane of Imprisonment was a legend even to the gods. According to its legend, an irredeemably evil creature and its servants had been banished from all of reality to the Negative Material Plane by the gods that came before the gods, with this demiplane housing remnants of their forces.

The color of this demiplane's ethereal curtain was unrecorded. Travelers were recommended not to cross color pools of unrecognized color, lest they accidentally find themselves here. Most realms had similar legends about a demiplane of this nature, with the local theology substituted, having all come together to imprison there what remnants of the evil force they could not or would not completely annihilate.