Xi is the letter of the Greek alphabet given to the first of the five sects in The Stronghold. Their members reside in a very small area of the stronghold, spanning about twenty rooms. Their inhabitants total eighteen, and they are made up of ten humans and eight Anthromorphics.
They are almost completely isolated from the rest of the stronghold, as they are the highest up. They can only be reached through a staircase about forty metres high. The bottom of this staircase comes out midway into a gargantuan cavern, rendering any hope of getting in or out, as the fall is enough to kill anyone. From examination, it is possible to see that the stronghold continues directly below it, at the bottom, however you can also see that there is a space where the stairs should end. It will only become possible to connect once a method is devised to allow travel through this cavern that does not pose a danger to the inhabitants and allows for two-way travel.
This particular part of the stronghold is unique in that it has three library rooms out of twenty total. The first two are the same size, the usual size for a stronghold library, however the other one is about eight times the size of normal libraries. It is lit by chandeliers, and contains roughly 90,000 tomes of knowledge dictated by the humans and recorded by the Anthromorphics.