This is the legendary Pied Piper of Hamelin.
Hired by the mayor of Hamelin to rid the town of rats, he was refused payment for the completed job.
Taking his revenge, he used his magical pipe to lure the children away from
the town as they danced to the music and followed the piper and his music as he did the rats before.
Depending on the version of the tale you hear, he either led them to a river to drown,
or led the children back to town after being paid the money he was due.