Taxman66, are you sure that the gem underneath the Obelisk in area 32 of the Moathouse is not a Black Cyst as well? The bottom of the well is extremely cold, which usually marks the presence of Tharizdun. The Yeth Codex handout on the BotB indicates that there can be more than one Black Cyst (" It is believed that all of the Masters' greater temples had their own Cyst, which they each used for differing purposes"). The gem must be viewed in the light of a torch of revealing, similar to the levers in the Outer Fanes. Also, Tharizdun speaks to the characters who enter it. It is far less secure than the entrance to the Black Cyst in the OF (room 21, description of the Cyst is on 21A), but I still think that it matches all the characteristic of one.
I just read the entry in the Moathouse for the scroll that contains the info about it. The scroll was there to foreshadow what the PCs will find later in the Outer Fanes, but I thought that this was a Black Cyst, too. Since my own PCs haven't figured it out yet, maybe it's better to leave the thing in the Moathouse ambiguous.





