Biocores: The Legendary Weapon Designer - Chapter 54: Boat to Xerath

The world that nurtured the civilization which reached the pinnacle of magic was on the verge of destruction.
The last survivor transmigrated to the past, when the magic civilization started growing, as countless mages were still fumbling their ways through the thorny paths of magic knowledge.
He, who came from the end of the magic era, aim to reach the pinnacle of magic.

The second prince has come of age. To celebrate the advent of such an occasion, the search for a suitable bride began. My younger sister was expected to go to the Royal Palace as a candidate. However, despite this, she eloped the day before her departure. I, the older sister she left behind, looked almost exactly like her – so much that we were often mistaken for twins…

One month, right? For just one month, shall I substitute in for my sister?

“You’re not taking me to bed. Ever.”

Before Li Daniu had time to be excited about being a prince who was about to ascend the throne, he learned that his country was about to be submerged by sea water.

“Can I wear it back?”

- Description from novelbuddy