Briefly about Saikyou Mahoushi no Inton Keikaku manga:
100 years ago, demons suddenly appeared and attacked humanity. At that time, normal weapons could not even scratch demons which caused the human population to decrease drastically. In order to protect themselves, humanity merged into seven countries. They managed to halt the demon’s invasion by their army implementing and using magic, but even now they battle with the demons to ensure the survival of humanity. On the forefront of the battlefield constantly risking his life, Ars Reigin finished his military duty at the mere age of 16, and requested his retirement. But the country cannot afford to let go of him. This is because among 100,000 mages, Ars stands at the pinnacle of them all as one of the single digit number 【Single Mages】.