Who Was the Gray World Saved For?

At the moment a young girl faced death, she was summoned to another world. From then on, she and the person who summoned her became master and disciple, and her master granted her a new life. The only thing her master asked of her was that one day she would kill him. His only explanation was that it was for this world, for the girl's world, and for... him and the girl herself. So what would the girl ultimately choose? Or was it all just about making a choice?

This novel and its characters are entirely fictional. Any resemblance to actual people or events is purely coincidental. Do not imitate.