Chapter 1: The Drop Off

Summary:
The Mers and their murderous cousins, the Siren, have always been at odds but recently something, or someone, has begun to rally the normally solitary predators under a single banner and the climate of relative peace on the reef where Harry was born and raised has descended into the tense anticipation of war. But juveniles will be juveniles regardless of what battles might be brewing, and unfortunately for Harry-at least according to Luna-there's a Siren in his future.
Of course, what this predator in particular wants from him isn't food.

found it in ao3 https://archiveofourown.org/works/9885926/chapters/22167107