Yuuki Yuuna is a Hero: Washio Sumi's Chapter 3 - Promise

Set in the year 298 of the Divine Era, three elementary school girls Washio Sumi, Nogi Sonoko, and Minowa Gin are entrusted with a mission of grave importance. They are destined to become heroes and confront Vertex, a veiled adversary attacking Shinju-sama, the god tree that protects Shikoku, the world's sole habitable region. Unaware of the true cost, they soon learn that this battle will exact a price greater than they could have imagined.