Washio Sumi wa Yuusha de Aru

In the year 298 from the era of the gods, elementary school girls Sumi Washio, Sonoko Nogi, and Gin Minowa are tasked with an important mission. They are to become heroes and fight Vertex, a mysterious enemy that is attacking Shinju-sama, the god tree that protects Shikoku, the only area in the world that is still habitable. What they don't know, is that this fight will cost them more than they could have ever imagined.
Note: The first of two manga adaptations, as this serialization was eventually cancelled.