MTL - Quest Master of Konoha - Chapter 451 Arena, 10th floor.

Ji Qingqing was at the end of her rope. When she was on the brink of wretched poverty, she received a system. The system told her; Lu Lixing came from a family that was rolling in money, but he was coming to the end of his allotted lifespan. If she married him, she would gain his enormous inheritance.

Lu Lixing was seriously ill. Just as he was about to die, he gained a system. The system told him: Ji Qingqing was young and beautiful. She was rich in vitality. If he married her, he could continue living.

Ji Qingqing happily came to Lu Lixing’s sickbed.

And then, Lu Lixing woke up.

***

Lu Lixing’s friends discovered that after Lu Lixing woke up, not only did this person that had always kept his distance from women gotten married, he spent every day being super clingy towards his wife.

Young Lu, can you be a little less embarrassing and act your age? Will you die if you take one step away from your wife?

Lu Lixing clutched his chest. His little heart had almost stopped beating because Ji Qingqing had left him for half an hour. He weakly waved his hand. He couldn’t be separated from her, not for the rest of his life. She was his life.

- Description from Novelupdates

Damien Julian is an 18-year-old loser, who has been bullied and mistreated his entire life, at home, he has to deal with his abusive father, and at school, he is constantly been bullied by delinquents, and to make things worse he is always looked down upon by his peers, till one day a mysterious screen appear before him as it said.

[The world has seen the lingering evil in your heart]

[Would you like to be one of the nine demon kings of this world]

[If so what are you willing to sacrifice?]

In the heat of the moment, Damien made a deal with the world, that he would give up everything he owns, or do anything it wants, and in exchange, he shall be reborn as a new Demon King, and what the world required from him as payment was.

[In exchange for power, become a slave of the world, and act as an anti-body for all eternity]

At that moment, Damien agreed, but little did Damien know, that becoming a slave to the world meant he has to be its immune system, what that entail is, that anything that threaten the world must be destroyed even if he has to wipe out humanity, to save it, or assassinate historical figures to bring fort balance or even manipulating events that can potentially end the world, to save the world, this is the story of a necessary evil, not the story of a hero who cares for others.

A mysterious and strange world, a human civilization lacking power, and a horrible and turbulent world of human life, Luo Kai survived humblely as a prisoner. Pain and suffering honed his will. War and killing promoted his growth. He suddenly looked back. Is already one of the most powerful beings in this world.

- Description from novelbuddy

Following a brutal and tortured death, a former assassin, known only by his codename Kuro, is given the opportunity to reincarnate in another world, a world with magic and monsters. Armed with an unrivaled healing factor and an inhumane pain threshold, Kuro opts to go for a reckless direct approach in this world instead of a stealthy and cautious one, and as a result, slowly gains attention bit by bit.

Having previously been a loner in his old life, he interacts and befriends people for the first time in his existence, however, his blunt nature also sees him making just as many enemies, both big and small. By combining his old assassin skills with the magic powers of this world, along with his high resistance to pain and rapid regeneration, he doesn't take long to establish himself as a formidable force in this world.

Initially, Kuro only faces minor threats, like weaker monsters and bandits. But before too long, the threats slowly but surely become more and more severe…from Zombies to a bloodthirsty empire to Vampires and Demons, and even divine beings…however, as the enemies he faces grow stronger, so does Kuro, gaining rare powers that have never or rarely ever been seen in this world…