マーメイド・ゴシック

Miletis, the country at the bottom of the ocean in which the merfolk live.
It was once a beautiful blue, but now, it is as black as a wicked darkness.

The protagonist is a mermaid princess.
When she was a child, she committed the crime of saving a human's life. As a result, she is incarcerated in a tower.

The months turned to ten years. It was then that her father, the king of the merfolk, issued an order in exchange for her to be free.