The one single goal in life of the Strongest Old Sage is to subjugate the Demon King. In an epic final battle, both of them met their mutual destruction at each other’s hands. However, the Demon King was not completely destroyed!
In order to destroy the Demon King once and for all, post-death, the Old Sage trains in the World of Gods and reincarnates as the Apostle of the Gods, Will. In the reincarnated future, the Old Sage had become a legend and his former disciples hold the supreme authority in the world. The disciples had created an organization to oppose the Demon King and an academy to nurture the new generation of heroes.
Thus the Apostle of the Gods, Will (8 years old), enrolls into that Academy of Heroes. This is the story of a boy on a mission to save the world through epic battles while enjoying academy life.
(Source: Shousetsuka ni Narou, translated)