Shadow of the Phoenix

Nadya and Shay have built a quiet life together away from the island city-state of Storm's Quarry and their outlaw vigilante identities, the Iron Phoenix and the Shadow Dragon. When that idyllic life is shattered by the arrival of desperate news from home, Nadya and Shay make the difficult choice to return to Storm's Quarry.
They find Storm's Quarry razed, the blood of the Duke drenching its stone, and the fragile peace with the powerful Kingdom of Wintercress destroyed. With their home in need of its masked protectors once more, Nadya and Shay join the resistance, infiltrate enemy lines, and seek the aid of an old foe in a mad plan to save the city that endangers both their lives and their future together. But in the final battle for the fate of Storm's Quarry, even their powers may not be enough.