It's an old saying, taken from the fairytale of the Pied Piper of Hamelin. You know, the town had a rat problem and they paid a dude with a magic flute to make them go away. Of course, when they didn't pay, he led the children of the town away. In Guild Ball, the Ratcatcher's Guild is being led by just such an individual. Piper's his name and he's also got a magic flute that will also cause players around the field to end up in spots they weren't originally expecting. Take a look at his cards in this preview.
From the post:
Piper was one of the earlier character concepts that the development team came up with for the Ratcatchers. We knew that we wanted to give the Ratcatchers a more ‘indirect’ playstyle, which meant that the guild needed someone to lead them who isn’t as directly powerful as a captain like Hammer, Shark, or Fillet. We also knew that including a character based on the Pied Piper of Hamlin in the Ratcatchers was an absolute slam dunk. As a result, in the Ratcatcher’s Guild, the Piper uses his melodic tunes to distract and taunt the opposing team as well as inspire his own into feats of incredible agility. When on the same pitch as the Piper, no one really knows who is making the decisions except the Piper himself.