He Had Been Planning This

Queen Nerida was a born ruler. Unfortunately, she had also been born female. The throne in the kingdom of her birth went to her younger cousin solely because he was a merman though she had been the merking's daughter. She had never gotten over the injustice. 

She tried to deal with it as best she could when she had gotten married off to the crown prince of a different kingdom. She had personally ensured that he became king so she could be queen by luring sharks to attack the former merking. 

No one ever figured it out and Nerida felt powerful. She might not be the ruler but she was still the most powerful mermaid here. 

But good things aren't meant to last. After more than a century of not giving birth, concubines were brought in. One of them gave birth to a prince before she could and she was furious because her role as merqueen wouldn't be secure if a concubine's son sat on the throne.