Waterfire Saga, Book One Deep Blue

In 'Deep Blue', the first book of the Waterfire Saga by Jennifer Donnelly, readers are introduced to Serafina, a mermaid from the Mediterranean Sea. On the day of her betrothal to Prince Mahdi, Sera is troubled by ominous dreams that signal the rise of an ancient evil. When her mother is attacked, Sera embarks on a perilous quest to find the assassin and prevent war among the Mer nations. Throughout her journey, she seeks out five other mermaid heroines to unite against the looming threat, forging a powerful bond of sisterhood as they uncover a conspiracy that endangers their world.