One Piece: I am Imu - Chapter 13: Doflamingo’s help!

As the second daughter of Ningyuan Army General, Qin Wanru, who had been set up by her so-called mother and elder sister, lost her reputation and suffered a lot on her wedding in her previous life.

The day the city fell, Feng Yun was treated by her father as a spoil of war and presented to the enemy general.Everyone pitied her, soon to be a captive, plunging into a fiery pit.Yet she veiled the little donkey cart that took her out of the city tightly, daring not to let anyone see the secret delight in her heart…As a child, she behaved oddly and spoke startling words, once predicting a war in which the whole army was wiped out, nearly being burned to death by her clan as a demon.When she grew up, her beauty was out of the world, but she was rejected by her husband's family.In the period of chaos and disorder everywhewe, where could a female captive go?“I do not seek a devoted partner to grow old with; I merely wish to rule unrivaled in this life.”In her previous life, Feng Yun was always the one being scorned by others; in this life, she decided to strike first, turning the tables on each and every one of them, those so-called unreachable flowers and beasts in gentleman's clothing, those degenerates… all to be scorned in return.

He died on the wedding of the girl he had a crush on, after a long and successful career. After convincing Yama to send him back to the world, he decides to fix the areas in which his life went wrong. Realizing the lack of love and romance in his past life, he sets out to conquer the hearts of various women.

- Description from Novelupdates

The strongest Demon Lord dies, and reincarnates as a human. Not only that, he because of various incidents turns into the hero’s best friend.

Follow his journey as he tries to help the hero escape his fate, and on the sideline to conquer the continent while he’s at it.