Supreme Monarch - Chapter 12 C12. Magic Crystals

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

While I was reading the first volume of “Danmachi,” I suddenly felt my cock grow. It grew so much that it started to hurt.Suddenly, my sperm gushed out. Before I could understand what was happening, I realized I was in a familiar place.

This is a century glittering with the brilliance of human civilization, a world where steam engines, magic, and demi-humans coexist, a chapter of complaints committed due to exploration.'Red Dragon Queen rises first, annihilating all human affairs with her wrathful flames.''The mysterious child of the sea will raise great waves to erase the sins of humanity.''The god of the skies will leave no refuge for the remnants, nowhere to belong.''The immortal witch composes epitaphs for them with her magic.''And I, will write a new chapter for this world'... ...... ...Years later, Fisher, who has received the apocalyptic prophecy and the miraculous tool called the Demi-Human Girls' Completion Manual, wishes to be recognized as a pioneer in demi-human academia, a savior of human civilization, a dove of peace, and a messiah.Instead of being the person who is torn apart with the firewood knife, and the savior divided into pieces.

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.