Tsuki ga Michibiku Isekai Douchuu - Chapter 400: First Joint Work

After his death, Bern awakens 10 years into the past as the third prince of Kailis, the enemy country.

Zhong Yansheng lived for eighteen years. After accidentally falling into the water, he dreamed and found that he was living in a book. In the book, he was a fake young master who was mistakenly adopted by the Marquis. After the real young master came back, he {real} was not taken seriously by the family and was driven to a separate courtyard in the suburbs of Beijing. His parents only doted on him {fake}, and they turned the real young master into a black hearted man, and finally his family was ruined. After waking up, Zhong Yansheng learned the news that Master Zhen returned to the house three days ago, but the whole family only revolved around him {fake} who was sick.

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

“Hmph, my dumb big sister is in danger!” A mischievous grin spread across his face. “Hehehehe... this gives me the perfect excuse to finally use this treasure I’ve acquired but never bothered with!” He rummaged through his belongings, carefully selecting various items. “A bit of this... and this... and a dash of this.” Boom! The explosion of energy sent a ripple through the air as he laughed maniacally. “Hahaha! It’s done! Now, all I need is a compatible host for it.” Years later... “Did I overdo it?” he wondered in awe, watching as his creation wreaked havoc, conquering world after world with ease, as though stealing candies from children.