MTL - Game Design: It is Very Simple For You To Fill In? - Chapter 72 you can get the answer too

In her previous life, she was humiliated and bullied every day. Her relatives despised her and her friends played tricks on her! Her mother and elder brother died one after another early. She gritted her teeth and survived the adversity alone, but only managed to live for twenty-six years!When she opens her eyes again, she finds herself back in time twenty years ago. To swallow insult and humiliation silently and let history repeat itself? If everything couldn't be changed, what was the point of her rebirth?! Who can stop her now that she defies her destiny and her abilities are emerging? In front of others, she is an ordinary student with an indifferent personality, isolated by her classmates. Behind their backs, she works step by step to build a brilliant and extraordinary business network.

One day every human on the world suddenly receives a mail claiming to enact the World Salvation Project. Half in doubt Kurosaki Shion, a diligent university student and loner, follows the mail’s instructions, becoming one of a million Demon Kings governing his very own Domain in the new world.

*** This is a post-apocalyptic story containing many fantasy elements. It is a somewhat dark tale following Shion as he grows while aiming for the unification of the all too sudden new world. It has many game-elements bolstered by strategical thinking while at the same time providing funny moments and romantic encounters.

Life used to be so simple,

Full of false hope and endless beliefs in one’s ability in wooing that beautiful girl.

But in a single instant,

Everything as you knew it, plummeted faster than a spoken rejection.

What is real and false can’t be determined,

As the world slowly falls into the maws of the apocalypse.

How will our simple friend respond to the call; the summons?

No matter what he chooses,

Chaos SHALL Reign!

- Description from Novelupdates

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.