The living architecture of the city is quite a sight.
The great hidden city in the cavernous depths of the southern Mountains. Great architecture of twisted writhing tentacles line the streets. The ceiling to your world is black rock and dead root hanging high overhead. The very air is wet and moisture is abound everywhere. The sound of flowing water from a nearby underground river echoes through the valley. It's sound is almost invisible to you however, as it has always been present.
You find yourself now standing before the very heart of your community. The Pool of the Elder Brain is not a sight to be taken lightly. Many Illithid do not find themselves in this company until their deaths, when they become one with the great creature. You are honored.
It was just moments ago when the four of you stood before Zsiet'Nech, Ulitharid Aedileator of your community. He chose you for the coming task at hand, and before he is to give you your orders, as custom demands, you must meet with the Elder Brain for advice and judgement.
The atmosphere is thick with anticipation and silence. No words have been spoken in this chamber for over one hundred and seventy five years when the githyanki tried to lay waste to the great being before you. To utter even a syllable now would bring much dishonor and shame not only to the individual, but the community as a whole.
The city maintains trade relations with other Underdark settlements via an agreement with the Neogi, a spelljamming race of aberrant slave traders and merchants. The Neogi provide resources that the city cannot produce in exchange for umber hulk eggs and specially bred or trained thralls. The city also exports its excess crop of brainmoss, a staple provision of the Illithid diet which they harvest and trade with other lonely illithid outposts in the world and nearby worlds. The city is perhaps one of the last places in the universe that can still produce psionically charged brainmoss.