Kassar is a triggered spawn for the Ghoulbane Empowered or Gogas Afadin quests. "Rattle the bones" in the corner to spawn.
Make sure you're in the castle basement. Note: the bones clickie is actually in the room past the spawn point, near the uncontrollable drelocks, at loc -56 -28 -98. Also the bones will not work if you have both the Ghoulbane Empowered and Gogas Afadin quests in your journal. You must cancel one to spawn Kassar.
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