Death March kara Hajimaru Isekai Kyusoukyoku (WN) - Book 8 - 8-19. The Battle in the Arena

In the legends that were told, ten thousand years ago the twelve demon kings wreaked havoc upon the world, almost completely destroying it. Ten thousand years later, the late arriving Bella discovered that she had been screwed over by her reincarnation, the ranks of the Saviors are already filled and she has to change teams in order to pass the days. Having her gender changed into a beautiful girl… she endured that, not being able to join the righteous side and obtain cheat abilities… she endured, but what she could not endure, was that the world that she had transmigrated to was toxic beyond belief. No one here does their rightful jobs properly, be they heroes, demon kings, or creator gods… in fact, Bella felt as if they all had also started with the wrong faction!

Xu Yuan transmigrated to a parallel world where everyone became lords. Initially, he thought he would be a lord too. To his surprise, he became the Dark Demonic Dragon, a starter hero awaiting to be summoned by a lord. He also awakened the Dragon God Cultivation System.

[Transformation of the Dark: You can level up quickly by staying in the dark.]

[Slumber of the Dragon God: For the Dragon God, sleeping means residing in the dark.]

[Rejuvenation of the Demonic Dragon: You can revive using the power of the dark. Your basic attribute points will increase every time you revive.]

[The Blade of the Hidden Wings: This is a dark attribute AoE skill. It can annihilate all enemies within the area of effect.]

[Unlimited Devouring: The skills of your enemy shall be yours now. Unleash this skill to devour your enemy and acquire their skills.]

…

Su Wan, a beautiful lord, had no doubt that she could easily become the most powerful lord in the universe.

She felt her heart beating fast when she first saw her starter hero dazzle amidst the flashing summoning light.

Unbeknownst to her, her nightmare was about to unfold.

While other heroes devoted themselves to training day and night in order to level up, her hero goofed around and showed no effort to improve himself.

While other heroes charged into the enemy’s base and valiantly killed off their foes, her hero hid in their base and slept soundly.

…

As time passed, Su Wan slowly discovered that there was more to her hero than met the eye.

Li Xuehao found a secret of monasticism in the cover of the genealogy. After spending ten years, he finally succeeded in building the foundation. Unexpectedly, he was eager to get rid of the evil spirits. After his death, he was born again in Yokohama, Japan, and was attached to Sakura, Tsurumi. The story of the high school student Gao Zhonghao in the college, the story begins here…

- Description from Shubaow (Translated)

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