This task is a pain, so I thought I'd share what helped me complete it.
Apothecary Cadmael says 'I believe that it is a sort of substitution cipher judging by the separation of the letters into word-length pieces. However, it is also clear that the code is more complex than a typical alphabetical shift. Ah, now I recall! A key is needed, some sort of cipher phrase that can be applied to the passage. If you can figure out the decryption key, you should be able to use it to change this note into sensible text. Speak the decrypted words aloud in the proper area, and you may be able to determine who or what is spreading the disease!'
He doesn't say it outright, but it is a . You can read up at that link as to how the key works, but it's easier just to try the . The settings I used are as below (I left out my result so as not to completely spoil it ):
When you have the decoded phrase, /say it aloud at loc +422.6, 0, 18.55. This is in the center of the circle on the floor of what is being called the ritual room. Here's a look:
If you'll notice, we cleared some room first. I suggest you do the same, because your /say will have an effect.