The Most Powerful Wizard's Scheme of Seclusion

The Most Powerful Wizard's Scheme of Seclusion - 　One hundred years ago, demons appeared out of nowhere and began to attack humans. At the time, conventional weapons could not even scratch the hard bodies of these monsters, and humanity quickly reduced their numbers and annexed them to seven nations, as if to protect itself. They have succeeded in thwarting the invasion through the use of magic technology, but the battle against these strange monsters still hangs in the balance, and the survival of the species depends on it. Ars Regin, who always risked his life on the front lines, asked to leave the army at the young age of 16 because his service was over. But there is no way the country can let him go. Because Ars is a single-digit number [single wizard] who reigns at the top of the 100,000+ wizards.　I think it's a royal fantasy.