Toaru Majutsu no Index - Volume 5, Chapter 3: A Certain Misaka’s Final Signal. Tender_or_Sugary.

Wu Lai was an ordinary high school student. One day he was struck by a flash of lightning from a different world, and suddenly entered a fantastic world full of magic and swords. Since then, he started a totally new life. In this supernatural world, he had his ups and downs, experienced numerous battles, subdued powerful evil beasts, and fell in love with a belle. His nickname “Demon King” was remembered throughout the ages there.

Stay or return was a question. In the face of this choice, Wu Lai, with the nostalgia and gratefulness for the original world, finally chose to stay…

Here was the legend of “Demon King”…

- Description from Novelupdates

After I defeated the Demon King, I was executed for false accusations by the heroines.When I came back to life, this was the first thing they said.“I know it’s shameless, but would you help us one more time? Without you, the world will be destroyed.”…Would you help them if you were in my shoes?

In a future timeline, the earth was visited by a species known as the Slarkovs.
Having lost their home planet and in search of a new one, earth was the next habitable planet for them.

Humans and Slarkovs made a deal with each other in return for the Slarkovs living on earth.
The Slarkovs traded their technology and knowledge for a new home.
They were similar to humans except for some of them who had slight differences so fitting into the society wasn't a problem.
Over the years Slarkovs and humans began to mate with one another and reproduce offsprings.
This in turn created a new species known as the mixed-blood.
Centuries later mixedbloods could tap into their bloodline and perform unimaginable feats.

Gustav born into an age where bloodline determines everything, struggles to fit with a useless bloodline.
His bloodline only grants him the ability to change his hair color, destroying his hopes and dreams for a great future.
His fate is turned around when an unexpected situation leads to him finding a system that grants him the power to unlock and upgrade bloodlines by completing quests but he's baffled by two quests with a five-year deadline. Success will grant him unimaginable rewards...
But only one thing awaits him if he fails... Death!

Follow Gustav on his interesting journey filled with unprecedented adventures, danger and death, maybe?

Marry him for a contract, but it is love that supports them to live together.

In order to raise medical expenses for the seriously ill father, she had to sign a contract with a strange man, she would be his wife, and he was responsible for paying for the treatment of her father.

Life after marriage is very ordinary. He is the CEO of the company. He is busy working every day and occasionally has a look at her, but he is full of disgust. However, she fell in love with him little by little, and she was very sad when facing his indifference. Neither of them knew how to resolve the misunderstanding between them.

After experiencing the death of her father and an unexpected pregnancy, they finally opened their hearts, and sincerely, began a happy life.