Iona and I have fought many battles together since she became my companion and, protecting me, has frequently been driven to her knees. But always survived. She and I just fought a Forsworn Briarheart (right name), who had some magic going for him and who was very tough. We fought it out through several reloads, in which either I or Iona died. The last time, he drove her to her knees and came after me and I finished him off, only to see her head for his caged giant frost spider which finished her off. The spider couldn't get at either of us and I stayed away from its venom, but she didn't. I decided this time that, if she's not smart enough to stay at a distance and kill it with the glass bow I'd equipped her with, then I'd move on alone. But why is it the game allows them to survive sometimes but die others? (In my previous playthrough the same thing happened to Lydia, but I never left her dead; she was very special.)