I can return to the age of mysterious - Chapter 104 - 100 Ordinary People (4000) (Thanks to Book Friend

A man who was once called the world’s strongest agent ended up becoming a teacher after his retirement, to train a new generation of agents. After many years of training his disciples, he was killed at the age of 60 by a secret organization’s ruse and was reincarnated in another world with his memories intact. Though he was surprised by the existence of magic and the strange species in that world, he quickly adapted to his condition as a newborn and took advantage of it. He acquired special magic and gained a massive amount of strength thanks to his rigorous discipline, in order to reach his goal: Resume his career as a teacher which he left halfway through in his previous life. This is the story of a man, who, based on the memories and the experiences of his previous life, became a teacher who travels through the world with his students.

As a wandering soul trapped in endless cycles of reincarnations, Claus must decide what to do in his current life.

A mysterious natural phenomenon caused the Earth to be plunged into darkness. The world turned into an endless bloody chaos and mankind is on the brink of extinction…

A world from another dimension suddenly appeared in the Milky Way galaxy and collided with the earth. A spatial collapse occurred, which allowed the monsters crossing over from the other side to invade various countries, and the earth fell into great calamity.

The military was powerless against the invasion because modern weapons didn't work against monsters from another world. Under the desperate situation, a god-like figure appeared and bestowed blessings upon the humans to awaken their power. From then on, the earth regained its hope, and these extraordinary people were called Realizers.

Tenku Okuihara is a five-year-old boy who dreams of becoming a Realizer to protect the world and his family. But an incident changed his life goal from a hero to a grim reaper.