Magister Alminas:
You can get the next stone by killing the Archon of Halisha in the Four Sepulchers. You'll need help to gain entry.
If that proves too difficult, go to the Shrine of the Loyal and slay Halisha's minions. If you collect 700 of Halisha's Marks, the Archon will appear.
The minions include Grave Scarabs, Scavenger Scarabs, Grave Ants, Scavenger Ants, Shrine Knights, and Shrine Guards.