As I understood of what is explained, she writes the novel based on both of the futures she lived through before turning back time for a third time, that made her have to give up her memories of those futures as a kind of penalty, which was also why Sarah couldn't go back to the Korean body, so as a way to make herself remember it she wrote the novel as a warning of what she should pay attention to, believing that sarah would blame herself for how things turned out in it and it would lead to her trying to change the future.
It's a really convoluted explanation honestly, but I hope it helps smh, also this was all done under the assumption that 1 her apprentice would follow the same paths as the previous timelines and 2 that the power of ambrosia would smh end up fixing her soul (don't ask me how she came to this conclusion, this is the part that left me a little confused).
Guys I have a doubt. So her korean soul saw the future of destruction of the empire right, and then she wrote another future as flower of darkness? Or did she write the future she saw? Why does she blame herself for how the world is following the book shr wrote? How can she be so powerful as to bend the fate just by writing it?