Secrets of the Pied Piper 2: The Magician's Key

The second book in the Secrets of the Pied Piper trilogy finds Max, Carter, and their allies on Summer Isle battling new foes, relying on new friends, and fighting for stakes higher than ever, in a series New York Times bestselling author Adam Gidwitz called “a wild fantasy adventure.”
 
On the Summer Isle, siblings Max and Carter discovered the magical land where the villainous Pied Piper led the children of Hamelin centuries ago. But only Max made it back.
 
Now she is determined to rescue her brother. To do so, she will have to find the one and only key to the Summer Isle. That key is controlled by Vodnik, a dastardly magician and soul stealer who will stop at nothing to get what he wants.
 
Meanwhile, Carter must overcome his own troubles back on the Summer Isle. He’s been separated from his friends and left with a very untrustworthy companion: the Pied Piper himself.
 
Struggling desperately to find each other, Max and Carter encounter a bashful trollson, a daring elf, a seafaring hobgoblin, and the ever-loyal kobold Bandybulb. As their paths converge, they prepare for the most important quest yet: to send the children of New Hamelin home.