Anime: I Want Harem - Chapter 16

After his death, Bern awakens 10 years into the past as the third prince of Kailis, the enemy country.

I once believed I was the heroine of a modern fairytale, blessed with beauty, wealth, intelligence, and a powerful legacy. Men adored me, women envied me, and I was engaged to the most desirable man, Cole Fay. My life seemed perfect, a dream where I held the starring role. But on my eighteenth birthday, that illusion crumbled. The truth hit me hard: I wasn’t the protagonist, nor even the villain or a side character. I was just cannon fodder, a disposable part of someone else’s story. But when fate recognized the cruel joke of my life, I was granted a second chance. This time, I’ll write my own story, living for myself. And this time . . . I will never love you again.

Transmigrating into a world of cultivation, you find yourself at the lowest rung of the sect's hierarchy. Everyone wants to bully you to gain some benefits. Fortunately, there are always a few junior brothers and sisters who are 'generous and understanding', allowing you to improve peacefully and secretly grow stronger. Years later, unbeknownst to anyone, you successfully become a deity within the sect. This day, your master visits you at the sect's request.'The upper echelons of the sect believe you have been diligent and loyal over the years, and though your strength is mediocre, you are an honest person, so they've found a marriage alliance for you.''An alliance marriage?''Not exactly. The other family has a rule: marrying into their family means taking their surname.'Is this... becoming a son-in-law who marries into the wife's family?

This novel is also known as Master Fu's Mysterious Darling Wife

She is the The Young Miss of Military family Shen, her face is peerless, her temperament is cold, and she is her family’s treasure, but isn’t accepted as one’s child by her mother;

He is The Sole Heir to the top giant Fu Family, his power is unparalleled, indifferent high-born, the captain of the mysterious military force, and the youngest commander-in-chief.

Because of her grandmother’s wishes, she married him at lightning speed. She thought that it was a marriage that she needed, but she didn’t expect to be greeted with his extreme pampering.

- Description from Novelupdates