The Amazing Maurice and his Educated Rodents: (Discworld Novel 28) by Terry Pratchett
It's not a game any more . . .
Every town on Discworld knows the stories about rats and pipers, and Maurice - a streetwise tomcat - leads a band of educated ratty friends (and a stupid kid) on a nice little earner. Piper plus rats equals lots and lots of money.
Until they run across someone playing a different tune.
Now he and his rats must learn a new concept: evil . . .