The Extreme Alchemist of the Mighty Cauldron - Chapter 40 - : Demon Beast Core

MC (female protagonist) dies in a plane crash and wakes up in the body of a beautiful but corrupt and lascivious prime minister in ancient times, Zhang Qinglian. As circumstances decree, he (she) cannot reveal that the original owner’s soul is gone else calamity might occur. So he (she) decides to walk a righteous path, determined to help the lovely but simple little emperor rule the country. Not backing down after facing torture, not losing courage, destroying rebellions and facing off with scheming princes.

- Description from Novelupdates

King Grey has unrivaled strength, wealth, and prestige in a world governed by martial ability. However, solitude lingers closely behind those with great power. Beneath the glamorous exterior of a powerful king lurks the shell of man, devoid of purpose and will. 

Reincarnated into a new world filled with magic and monsters, the king has a second chance to relive his life. Correcting the mistakes of his past will not be his only challenge, however. Underneath the peace and prosperity of the new world is an undercurrent threatening to destroy everything he has worked for, questioning his role and reason for being born again. 

You shall become king—

Ruling over all “serpents” in the world…

Become the man who bears the brunt of dragonkind’s fear and hatred.

A modern era when humanity is threatened by attacks from giant dragons flying from beyond the sky.

Controlling “leviathans”—serpentine spiritual beings created through magic—humanity is locked in a protracted war of resistance against the dragons.

Fortunately, there exist witches known as “magi”—battle maidens of salvation who formed covenants with “serpents” in order to combat dragons.

Nicknamed Hal, the young man called Haruga Haruomi makes his living through facilitating the covenants of these girls.

At the (partially coercive) request of his childhood friend Asya, one of Europe’s top witches, Hal returns to Tokyo New Town after an absence of three years.

Accompanied by the beautiful Asya whose appearance rivaled a fairy’s, Hal is running all over the place to secure his base of operations and taking care of matters related to school transfer when the elite dragon Soth suddenly attacks—!?

Life has been breathed into a brand new myth between dragons and dragonslayers.

A grand opening to the “birth” of a top-class battle epic!

It was supposed to be a perfect deal in which both parties got what they needed. However, after the child was born, he tore up the agreement with discontent and told her, “Who said that we’d only have one child? I want twins! Twin brother and sister! You’ll continue giving birth to more children until you bore me a pair of twins!”