New Beginning - Book and Key, #4

The enemies of good King Adon of Uwd have regathered in greater strength for a final battle for the kingdom. The rebellion is led by Tumah, who calls herself a queen, and Zanah, whose dark magic enables him to change into forms as diverse as a dragon and a panther. With the armies of Zanah and Tumah coming against the King's followers, the heroes from Earth-Uncle Frank, Thomas, and Gracie-return to defend the true King's followers and, they hope, rescue some in rebellion against him.
The battle that ensues pits the enemy's forces against a ragtag group that's led by a boy inexperienced in war and a girl whose only weapon is song. They appear vastly outnumbered, but know that King Adon is with them, and that a victory in Uwd will accomplish something back on Earth more wonderful than any of them could imagine.