Decline of Ráglauth's Thrall

Looking at the Illithiad, it says that the illithids need one brain a month to survive.  Then there is a claim that the illithids had a capital called Penumbra which was absurd in size, being a diskworld built around a star and taking a millennia to build.  Putting the one brain a month and the impossible size of Penumbra's illithid population together, you have to think that the space around this "diskworld" must have been blotted out from all the brain-less corpses.

For the realm Bal-Kriav  -  the realm being all of what makes up the Cobalt System and the Quaratum System, Penumbra was a city-state of illithids dwelling on Ráglauth. 

Elder Brain

Ráglauth is one of the habitable moons orbiting Ghífthauk.  In the Lith-Crillion Era, eight illithid city-states reigned supreme over the moon's thrall population.  The thrall being any humanoid of any race except illithids.  The illithids dominated the moon and enslaved its humanoids.

Generations of in-breeding led to declining health and birth-rates in the thrall populations.  This led to wars between the city-states as they sought to seize their rival's thralls.  These series of conflicts are the Flayer Wars.   The victor of these wars were the Suellk.

Even with the captured thrall of their rivals, the Suellk could survive several centuries at most.  They knew this from the outset of the Flayer Wars, as did their rivals.  The Thinker and Creation Creeds worked on solutions, but only those at  Penumbra found a solution to their dilemma.  In the capture of a rival city-state, Penumbra's War Creed sought to capture the rival's elder brain.  Those that survived were then imprisoned in devices built from the innards of a still-living primordial.  These devices are  elder brain prisons called the Silver Mirrors.

The Silver Mirrors were used by the Suellk to leave Ráglauth and seek out fresh thrall.  The First Suellk Invasion and then the Second Suellk Invasion were made possible with the Silver Mirrors.