Washio Sumi wa Yuusha de Aru

In the 298th year of the divine era, three elementary school girls—Sumi Washio, Sonoko Nogi, and Gin Minowa—are assigned a critical mission. They must take on the role of heroes to confront Vertex, an enigmatic adversary threatening Shinju-sama, the sacred tree that safeguards Shikoku, the last livable region on Earth. Unbeknownst to them, the battle ahead will demand a greater sacrifice than they anticipate.