These Divided Shores by Sara Raasch

As a child, she committed unforgivable acts to free Grace Loray from King Elazar of Argrid. Now Elazar’s plan to retake the island has surpassed Lu’s darkest fears: he’s holding her and his son, Ben, captive in an endlessly shifting prison, forcing them to make a weapon that will guarantee Elazar’s success. Escape is impossible—unless Lu becomes the ruthless soldier she hoped never to be again.

Vex failed to save Lu and Ben—and that torments him as much as his Shaking Sickness. With the disease worsening, Vex throws himself into the rebellion against Argrid. The remaining free armies are allied with the stream raider syndicates—and getting them to cooperate will take a strength Vex thought burned on a pyre six years ago.

Imprisoned, betrayed, and heartbroken, Ben is determined to end his father’s rampage. Watching Elazar sway the minds of Grace Loray as he did those of Argrid, Ben knows he has to play his father’s game of devotion to win this war. But how can a heretic prince defeat the Pious God?