Master Tobald:
How narrow-minded! To get your hands on the fifth Stone of Commune, you must slay the Archon of Halisha! Find him in the cemetery or at the Shrine of the Loyal inside the Wall of Argos.
At the cemetery, you must enter the Four Sepulchers, which is farthest inside. At the shrine, you must kill as many low-level beasts as you can before the Archon will appear. That's all the advice I'll give you!