One Earth by Kristin Durfee

In the final book of the Four Corners Trilogy, Levi, Aura, Julia, and the rest of our group are split up and scattered across two worlds, but their goal remains the same: defeat Lady Grustmiener and secure the peaceful future of Earth. As they race to be reunited, new forces, both for and against them, come into play. Will they find each other in time, or will evil triumph once and for all?