Mattara (The Ventura Series, #2)

Rogue and Ranger managed to track down the first piece of the Heart of Ventura. Now joined by its Guardian, Elva, they have found their way to Mattara, an underwater world of sea creatures the likes they never thought to see outside of fairytales.
But Mattara is being plagued by a malevolent force. Hell bent on destruction, it has come to be known as the Destroyer, wiping out entire villages overnight.
Where is the Guardian of Mattara? Why aren't they doing their job and using the Heart to p...