Sevens Gaiden

Sevens Gaiden summary:

Its been thousands of years since Lyle Walts saved the world and brought prosperity to the land.
But now humanity, having failed to fight against the monsters and almost all of humanity dying, they were forced to construct full walled colonies and avoid all contact with the outside world in fear of the monsters that now are the rulers of the world…
This is the story of Leo Walts, a descendant of Lyle Walts the hero and ruler. This all started once Leo picked up a blue jewel, and the first thing he heard was… My name is【Lyle Walt】, I am your ancestor.