A Legend of Shadow and Light by Jordan Lee

After opening the portal to the Unseen Vale, Owen and Colt find themselves in an unforgiving world of spirits and monsters. They have no choice but to fight to escape the spiritual realm where the dark god Mordren resides. Not only has Mordren been freed from his tomb, but he comes with a fury and vengeance unlike any other for the living world.
Upon reuniting with Brom, Owen and Colt take on an unlikely ally—the Core Wielder, Elian—which causes tension within their group. As they journey out of Vanhelm, they face not only monsters released from the spiritual realm but also dangers from their own world that force them to band together and fight to survive.
Just when they believe they’ve escaped harm, Mordren threatens to claim Owen for a sinister plan that spans centuries. With their world hanging in the balance, Owen and Colt must devise a plan to destroy Mordren or risk losing everything they’ve fought for—including each other.