From the publisher:
Rats were not the real problem in Hamlin. It was zombies. They roamed the streets, attacking peasants, assaulting the stockades and even threatening nobles . . .
Until the Piper came to town. Just as the largest force of zombies Hamlin had ever seen began breaking through the barricades, the Piper arrived and somehow led them away. All he asked at the time was a bit of gold.
When the Piper returned for payment, the Mayor sent him away empty handed.