The Disappearance

They had no idea they would be heroes. Neither did the wizards who brought them to Thera.
Tom is sixteen when his younger sister, Jessie, vanishes before his eyes. Her loss, coming after an accident that left him in a wheelchair, causes his family to disintegrate.
But Tom learns that Jessie's disappearance was not an isolated event. Long ago, wizards created a rift that shifted Earth's once-abundant magic to a separate world called Thera. Now, a terrible war rages there: some wizards strive to reunite the worlds, while others believe doing so would destroy both spheres. Both sides are stealing children with magical talent, like Jessie and Tom, to aid their cause.
Desperate to bring Jessie back, Tom allows himself to be changed into a centaur so he can cross to Thera. Once there, he learns that he and his sister are pawns in the wizards' ongoing war. Rather than be manipulated, Tom and Jessie wage their own battles to save both worlds and return home.