Terminus prisons were magical objects created in ancient Estorium. On the outside, they appeared to be small, black pyramids, just barely bigger than a human fist. Inside, however, was a massive extra-dimensional space comprised of a sprawling labyrinth of hallways and chambers, contained within a chronal field that distorted and heavily slowed the passage of time. As their name implies, they were used to imprison criminals against Estorium - usually without the possibility of release. It is unknown how many terminus prisons survived the fall of Estorium after the Illithid War, but the wizard Vyrellis claims to know of the location of several.
Creatures within a terminus prison do not age nor require sleep, food, or drink, though they can be injured or killed. The interior could also adjust its layout to create new chambers to fit the needs of its various prisoners.
One of the more well-known terminus prisons, at least in modern times, is the Pyramid of Shadows.