If we think back to when Claude and Ivona went to the castle ruins, we don't really know HOW Ivona managed to free herself from getting taken over by the magic power. I think Claude made a deal with it—he took the evil twisted part of the magic power, while Ivona was spared and she just got to increase her magic. From what I've seen in this chapter the dark purple fumes look like the ones from castle and that could be why he can't wield the holy sword anymore...