Demonic Dragon: Harem System - Chapter 16: Revenge is pleasurable.

After a shocking reincarnation, our hero opens her eyes to a new (and messy) world the last thing she remembers is reading Lady Viper’s Vow, a dark fantasy novel about a cruel, misunderstood villainess and her path of doom. But when our protagonist finds herself reborn right in the heart of an attack, she’s moments away from a second, way-too-soon death. Just as she’s about to face the end, a mysterious “system” pops up with a single mission: Level Up or Die Trying. It’s bossy, snarky, and her new best friend. As she levels up into a borderline-cheater powerhouse, she discovers her fate isn’t to be a hero, a princess, or even a sorceress, but to protect the one person no one ever expected her to side with: the villainess herself.

In the twenty-ninth year of the First Emperor, Bo Langsha was attacked.

When Ying Zheng merged the memories of the 21st century history doctor, looked at the mighty mountains and rivers in front of him, and said to himself, “In this life, my Daqin can’t die!”

- Description from novelbuddy

Reiji Saeki, who lives in the north eastern countryside town, lost his life in an accident during summer vacation. ……But before he knew it, a mysterious ball of light hovered in front of his eyes in a world of whiteness. That ball of light was a magician from a parallel world, and is looking for a person qualified to inherit the magic they had created to prevent it from being lost. Reiji, who rode on that suggestion, now lives in the parallel world Elgin, with a new body and powerful magical power.

Xue Xi is forced to fall in love at first sight with this dangerous man on their first meeting. With death being the only other option, she makes the first move: