MTL - People Are In Marvel, Behind the Scenes - Chapter 340 Invincible Doom (below)

– The planet [Earth] has entered the abnormal state [Apocalypse].

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

Xue Xi is forced to fall in love at first sight with this dangerous man on their first meeting. With death being the only other option, she makes the first move:

Lan Xuan smiled gently as she said, “My dear sister, who told you to obstruct my path? Your man belongs to me, your dowry belongs to me, your everything, belongs to me.”