"Shuang'er", a girl who grew up in an orphanage, grew up to be a top mercenary for a famous organization in Country B. The undefeated god of war, she had an accident during a mission, and when she woke up, she was reincarnated in the body of a good-for-nothing concubine.
The idea: a female killer travels through time to become a general's concubine, and is reborn in nirvana.