The issue is that you ran the instance Taking a Stand with someone who had progressed that far, but hadn't actually gotten to that point yourself. There is a bug that is causing his phasing state to change for you as well, which it shouldn't until you complete the wrapper quest for Taking a Stand (at which point Thurbald, Elfrith, and Cuthric are dead.)

This is being fixed in the next patch. (Thurbald and his companions should automatically reappear for you at that point.)