Background Story to Susans letter.
Susan and Caspian had a child a few years after the end of the novel. She is a little girl who is called Harmony and she is going to be the next queen of Narnia. She cannot leave Narnia due to the fact that she was born there and that nobody outside of Narnia knows about her. Susan knows that Harmony will be kept safe due to the fact that she has been placed under special protection by Susan and her siblings. Harmony is 8 at the time of the letter being written and she has not seen Harmony since she was 2 just to a situation that arose concerning Harmony's safety if she stayed. Caspian is her father and doesn't see her as he doesn't want to see her.
