I have read a few merfolk book series in the past but this one is by far my favorite!
Book Nine of the Awakened Fate Series
The Beast is dead.
Long live the Beast.
After narrowly escaping death at the hands of the Judiciary, Ari found unexpected allies in the depths of the sea. With the mermaids and the Beast beside her, she chased her enemies through the ocean, only to discover a deadlier threat than she could have imagined waiting in the sunken ruins of a place that had been lost a thousand years before.
And in one moment, everything changed.
Now, the Beast is gone. The mermaid world is in tatters. Ancient magic has turned Ari’s worst enemy into a force unrivaled on land and sea, a monster more terrible than the Beast had ever been. Ari’s friends and family are in mortal danger, and Chloe and the mermaids are the only hope to save the wizards from total annihilation.
In the war against the Judiciary, the mermaids and wizards must join together or else the entire world will fall.