Summoner Noctisse:
You must defeat the Archon of Halisha to get the fifth Stone of Commune. You can find him at the Imperial Tomb or the Wall of Argos.
Enter one of the Sepulchers at the cemetery to find him, or slay his minions at the Shrine of the Loyal by the wall and he'll appear. You must defeat him to get the stone. Good luck, my friend!