Priest Bastian:
The Stone of Commune that you seek is in the possession of the Archon of Halisha!
He resides inside the Four Sepulchers, but you must not pursue him there alone! Form a party with at least three other players before even attempting it!
If you must pursue him alone, attack his minions at the Shrine of the Loyal. He will show himself to defend them when you've gathered 700 marks of Halisha.
His minions include Grave Scarabs, Scavenger Scarabs, Grave Ants, Scavenger Ants, Shrine Knights and Shrine Guards.
The Tablet of Vision that you seek can be found between the Wall of Argos and Goddard.