known bug, for now you can stil finish the quest by aoe attacking around the bridge, it wil reveal the invisible hag you can then beat her to death and talk to the girl. she's near the object that lowers the cage, if your camera is positioned so that the girl you save is north east of the cage the hag wil be a little bit west of the contraption you use to lower the cage. you can use aoe effects to reveal her
