Used by the Mind Flayers primarily as a holding space for various beings that they plan to use at different points in history to alter events as they see fit, the Durance is a strange place that bends the laws of physics and distorts reality.
A single hallway, forty feet on all sides, the durance extends for almost two miles. Alcoves in the walls contain items of power, alien creatures, heroes and villans alike, and any thing else the Mind Flayers see as being useful to them.
Around the alcoves and all across the floor and the cieling are etched glowing blue runes, the only light source. These runes are the history of the Illithid Empire, writen from start to finish. Reading the runes is imposible, because the runes themselves are an imposibility. Writen in the Aeonic, which has no writen form, they are a mystery to the Mind Flayers, who know what they are, and are currently attempting to find a way to decifer the runes.