Josef was a follower of Agrith-Naar in the Shadow of the Storm quest, and was one of the dark wizards recruited by Denath to summon the Agrith-Naar demon. The Demonic tome can be found on the ruined kilns around Uzer.
Josef ran away from the temple in the background of the quest and his friend Matthew is worried about his fate. What happened to him was that he turned to dust because of a spell caused by Denath.