MTL - I’m the Hero, His Brother, Wear It Quickly - Chapter 1 Demon Undercover【08】

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

Time is an illusion, and reality is a fragile thread in Kael's fractured world.Kael's life is defined by darkness, memories that flicker in and out like glitching shadows. Struggling with haunting nightmares and terrifying hallucinations, he finds himself trapped in a reality that doesn’t make sense—a reality where time is breaking, and nothing is as it seems.When a mysterious red notebook, filled with cryptic symbols, falls into his hands, it opens a gateway to a world where the laws of time no longer apply. Every hour, every minute, every second twists and warps, and Kael begins to realize that something—or someone—is manipulating time itself, pushing him to the edge of his sanity.But he's not alone. A girl, whose existence is as twisted as the world around them, seems to be connected to his suffering. Kael's bond with her grows as they both search for the truth behind their disturbing experiences, yet the more they uncover, the deeper they fall into an abyss of despair and madness.The Pulse, an unfathomable force that distorts time and reality, pulses through their very existence. As Kael inches closer to the truth, he faces a devastating choice—one that could either save them or shatter everything they hold dear.Time is collapsing, the Pulse is awakening, and Kael must confront the horrors lurking in the fractured timeline before it consumes them all.

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.