Head Summoner Radyss:
Young leaf, you must destroy the Archon of Halisha to get the fifth Stone of Commune. Hack... Find the devil in the Imperial Tomb or at the Wall of Argos.
If you go to the Imperial Tomb, you can confront him by entering the Four Sepulchers. If you go to the Shrine of the Loyal, you must attack his minions as you see them. Then the Archon of Halisha will appear. Keep in mind that you must destroy him to get the Stone of Commune.