I Traveled Back Before the Villain Destroyed the World

Please add this to your collection. "Bows repeatedly." I'll revise the blurb before the story begins, but the overall idea won't change.

This is a chuunibyou story about a female sword immortal who is forced to transmigrate back to the time before the apocalypse. She kills the young villain with a single slash, only to find that the world-ending outcome remains unchanged. With no other choice, she transmigrates back again and stays by the young villain's side to uncover the true cause of the apocalypse. After enduring separation and loss, she saves the world with a band of people from the lowest rungs of society.

Theme: A single spark can start a prairie fire.