Carl's musical skill, which lets him play any instrument and has nothing to do with rodents.
Rosetta suggested that Princess Donut's new Gather Ye, Vermin spell could be used in conjunction with Carl's Pied Piper skill. Carl corrected her twice over: he didn't get to keep Pied Piper until after the events she was thinking of, and the skill simply allows him to play musical instruments — it is not like the fairy tale and has nothing to do with rodents.
Donut seized on the implication that he could now play his bagpipes and proposed starting a band to play at Louis's Second Wedding after they saved him. Carl replied that the bagpipes live in his Probably-Never-Going-to-Need-This-Stuff Folder and are staying there.
Carl's musical proficiency skill, misread by others as animal control.