The Baby Prisoner in the Winter Castle

On the day when the country is defeated and all members of the royal family are executed, the illegitimate daughter of the beaten royal family unexpectedly becomes the last member of the royal family.
“You cannot put a person to death under the age of eighteen.”
Nine-year-old Clarisse was saved thanks to the law of Saffers, a victorious country that said that minors would not be executed.
“My older brother said he would kill me before I was twelve. But to be able to live to be eighteen, I am very happy.”
And Maximilian, who raised young Clarisse safely until she was 18 and had to be put to death.
“Do not forget. That the day that child turns eighteen, you must bring that head to me.”
The two of them head to the deserted land, the cold northern winter castle.
A grace period of 10 years given to Clarisse.
What will be C larisse’s fate?