Few (sane) adventurers are aware of the frightening alien intelligences that live on the other planets. Even those that doe are generally unaware that some of these creatures have access to planar travel as well as interplanetary travel. The maddening brain-stealing fungi from Yuggoth are the perfect example of something no adventurer expects to run into on the Astral.
So, what better place to experiment on captive brains?
This free-floating structure is controlled by a massive collection of brains that the Mi-Go have assembled into some sort of brain cluster. A few Mi-Go may be present at any time, as well as a few orderlies (typically strong creatures like ogres who have had Mi-Go brains grafted into their heads). The rest of the population are a mix of kidnapped people lucky enough to still have their own brains; kidnapped bodies used as a chassis for other kidnapped brains; and a few classic βbrains in a jarβ to round out the mix β all subject to horrifying and bizarre experiments testing the limits of the standard human / elven / halfling / dwarven brains.
https://rpgcharacters.wordpress.com/2018/09/07/astral-sanatorium-of-the-mad-mi-go-brain-cluster/