The Testament of Sister New Devil Temporada 1 Episódio 4

Basara suffers a deep wound in the battle against the white-masked man after Mios power. Mio regrets getting Basara involved, and challenges the white-masked man to battle on her own. Meanwhile, in order to protect Mio, Basara resolves to use the forbbiden art he sealed away after the events of his past. What was the tragedy that visited the hero village and Basara?