Guard Bayard:
To find the next stone you must defeat the Archon of Halisha inside the Four Sepulchers. To enter will not be easy. If you can't form a strong party, go to the Shrine of the Loyal. There you'll find Halisha's subordinates. Collect 700 of their marks and the Archon of Halisha will appear.
They include Grave Scarabs, Scavenger Scarabs, Grave Ants, Scavenger Ants, Shrine Knights, and Shrine Guards.