In This Life, I Will Be The Lord - Chapter 37

As a promise, he went out into the world as an assassin. 3 years later, after becoming the Supreme Assassin, he gave up on everything he had and returned to the ranks of ordinary people. But one day, Kismet, a mystical game created by the National Research and Development had appeared.

Enigmatic hands, spectral frame, and one cunning strike just like the swing of the Grim Reaper. The Supreme Assassin takes on the mystical world, and how his legendary tales sets off, progressively ascending the altar to becoming the best.

- Description from Novelupdates

Collapsed without warning, and upon awakening, he found himself back in his childhood.He thought it was a blessing from above, but unexpectedly, tumultuous changes ensued, shrouded in mystery...

their example, leading to continuous warfare! However, the aboriginal people of this otherworld who have been blinded by the aesthetics of violence! Rightnow! And in the future! Let me teach you what kind of mecha represents the romance of a man! Let me show you what kind of propulsion is the iron torrent! Let me instruct you what kind of weapon is the deterrence for peace (implied by “nuclear” here)! Let me! Teach you! What exactly is the air-to-ground cluster offensive and the all-round air, sea, and land strike!! Gentlemen! I, indeed, love war! But what I love even more is to stop war with war and bring about a prosperous future where the whole world is unified for thousands of generations! Unite all nations and subdue all races! When the whole world is united as one, it will last forever...