Horror in the Appalling Hills

From the introduction: You have been asked to go to Green Village on the eastern most frontier of the Empire of Bonparr to meet with this person and to go into the hills near Magnatap to investigate his claims. The mysterious man, known only as Grax, has exposed and helped to capture an "alien" spy in Green Village. A local humanoid merchant, named Fogner, had had his brain removed and "replaced" by an intelligent mobile brain creature. The body and "brain creature" is still being examined by members of the Healers in Jainus.