User blog comment:15sok/How happen to angels, devils, fallen angels, and gods when they died./@comment-1359322-20131216052129

It depends on how they die, in some cases, their souls can be completely terminated, in some cases, their soul pass on the the supposed place. In some cases their souls become caught. But once a Devil, Fallen Angel and Angel die, they die, no reincarnation or anything for them. Issei was an exception that he was lucky to have Ddraig and the past possessors protect him while Great Red + Ophis provide a new body for him.