Makai Youjo ni Tensei Shita Oji-san wa Heiwa no Tame ni Maou ni Naritai

An old man was laid off from his job at a black company. While wallowing in the depths of his despair, he encounters a mysterious stone that sends him to another world filled with magic and alchemy. The man finds himself reincarnated as Hiyo — the youngest daughter of the demon lord.

Hiyo attempts to make a dazzling fresh start in life by utilizing both her newfound magical powers and prior tribulations as a salaryman, however, the demon world is in the midst of a battle for succession. In order to change herself, the ex-old man is determined to defy the demon lord and her siblings. And after meeting Ernest, a hero with his own circumstances, they work together to take over the throne of the demon lord — for both the sake of peace and to search for a place to belong. The two of them defy destiny, even if it means destroying the world to do so...

(Source: Cmoa Comics, translated)

Note: Includes three extra chapters.