Maoyuu Maou Yuusha - Volume 2 Chapter 3, “For the Sake of this Land”

In her past life, Michelle was an ordinary doctor who worked in a small hospital, but once in a while, a strange envelope would arrive at her apartment and she would take up a weaon and turn into a deadly assassin known for her stealthiness. But being a top assassin, Michelle had many enemies and ended up dying, only to wake up in the body of Michelle Grace, a pampered young miss in a world similar to ancient dynasties, except that cultivation existed there. Not everyone could cultivate, but Michelle Grace could.The original Michelle Grace was engaged to the Crown Prince, and he killed her before Michelle took over the body, so after making a deal with Michelle Grace's father, the Archduke, Michelle took on the name Grace and started a revenge plan on the Crown Prince, his lover, his mistress and his father, the Emperor.A bit troublesome: Michelle gained the attention of two men who together were the Dark Lord. They were horrifyingly powerful, so Michelle had to be careful around them. Luckily, they didn't have in their plans harming her, as they were intrigued by her constitution. Michelle possessed a rare blood, which they could use.

Darkness. The ultimate ruler, the true One-Above-All. Everything starts from it, and everything returns to it eventually. People say that in your final moments, darkness seeps in your very existence, engulfs your whole being. They say it’s an incredibly relaxing feeling that takes you on your journey to your afterlife. They say that in the very darkness, the ‘nothingness’ your weary soul will finally be able to rest and relax. This brings up some very philosophical and existential questions. Do you even exist?

Follow the epic journey of Aiden, the last primal, a unique and ancient race as he finds his place in a strange, new world, filled with threats and violence.

“You mean that Prince Yu wants to marry me?”

“Exactly, young master, Prince Yu wants to marry you!”

“Hell no! Is he out of his mind? Even if woman is a rare species nowadays, as the prince, he could have a chance to marry one. Is it because he got excluded, or maybe he has sexual impotence, so he could only marry a man?”

“No, Prince Yu is charming, majestic, endowed with civil and martial virtues, and also in perfect health.”

“Then why would he marry me?”

“Maybe because of your reputation out there.”

“Reputation? Am I quite famous?”

“Of course. Young master, you’re far-famed.”

“Really? What did they say about me?”

“They said you are ugly, good-for-nothing, short-lived, a freak, with no morals, and shameless…”

…

- Description from Novelupdates

The story is about a a hitman, Who dies because of his own recklessness and transmigrates into a game he used to play after his retirement as a hitman.I'm not gonna spoil any further, but i might answer few of your questions.Is MC overpowered?Nope, he'll become strong over time and all battle he'll fight will be because of his effort and techniques.Is there magic and adventure?Yes! It have both magic like normal isekai with adventure and fighting with monsters and many other things. There are other important things like “Essence Stone” But let's talk about it in the chapters.There will be harem?Not really, He will be in contact with many beautiful female and males but one waifu but i haven't decided yet.All i can say is, give it a try, I'll try my best to make it worth your time.