Da Tang Shuang Long Zhuan - Book 63: Chapter 10: Reign of Zhen Guan

Tristan never thought his life would lead where he would be signing all his freedom away, makeup masking his true identity, and walking toward a bond he never agreed to. All to protect his family and survive a world that thought little of him.Alpha King Mitch is powerful, feared, and completely uninterested in love. For him, this is just another duty—an arranged union meant to quiet the pack elders. But when their eyes meet, something shifts. Tristan feels it, even if he’s terrified of what it might mean.Tangled in secrets and bound by a lie, Tristan must navigate a fragile dance between duty and desire… all while hiding the one truth that could destroy everything.

When an S-class elite assassin-raised captain of a special ops suicide squad comes home from a high-stakes Africa mission, he finds himself offered something that could potentially set him up for life- a curious assignment that he could use to finally exit his days of war and slaughter.

Then placed into a high-school setting, Lin Yi is tasked by the chairman of a top-tier multi-billion company with bodyguard duty for non-other than his own daughter, a certain little Miss of said high-school.

His enemies, once ranging from drug lords, mafia bosses, political figures, and national leaders, are soon replaced with spoiled kids who happen to have rich papas and mamas, and Lin Yi finds his intense expertise and high-threat skillset misused, and very underwhelmed…

But he does get a harem cast as diverse as his old enemies, so Lin Yi decides that, for the sake of messing around with girls as an invincible entity in a down-leveled environment, leaving his bloody and death-filled war days behind is something he’d definitely want to strive for.

- Description from Novelupdates

After the nuclear warfare, massacres started to spread throughout the entire world. The order between humans completely collapses, and the strong eats while the weak gets eaten becomes the #1 rule.