Rise of the Mad God. The Sommerland Chronicles, #3

There is no victory without sacrifice. Emma Thurston was never meant to become the enemy. After surviving multiple journeys into the war-torn world of Sommerland, Emma falls under the control of a long-banished Mad God whose return threatens both realms. Now, as her unwilling vessel, she leads a rising force of destruction that no army can withstand. Her brother Ben refuses to give up on her. Teaming up with Sommerland's future Empress and a fragile alliance of rebels and magic users, Ben must do the impossible: stop a war that has raged for centuries.
and save the sister who may already be lost. But as Emma's true lineage comes to light, the truth threatens to shatter everything Ben thought he knew about their past-and their connection to the worlds now on the brink of collapse. Because if Lucillen cannot be stopped, Sommerland will fall. And Earth will be next. In a final battle where love, loyalty, and sacrifice collide, Ben must face an unthinkable choice: save his sister.
or destroy her. The epic conclusion to The Sommerland Chronicles.