I forgot the title of this novel I read a while back. It goes like this. FL is born in an aristocratic family which has a bad relationship with her and is turned into a sword for the royal family. The emperor uses her for around 400/4000 years in their battles. Then she goes back to her original body. Since she was fighting for such a long time, she has absorbed mana from her users and has a extremely large mana reserve. She then goes to tutor a prince.