Master Felix:
Let me tell you. The devil Halisha made the Imperial Tomb its home during the Elmoreden era. Feudal lords paid tribute to dead emperors there.
One of the devil's minions, the Archon of Halisha, has the stone. It can be found somewhere inside the Four Sepulchers.
If it proves too difficult to enter the Sepulcher, there is another way. Go to the corner of the Wall of Argos at the entrance of the grave, and find the Shrine of the Loyal.
The Archon will appear after you slay about 700 monsters at the shrine.
The stone tablet matching the Stone of Commune is at the corner of the Wall of Argos.
Either way is treacherous, but I have faith in you!