"Smash" star Andy Mientus is set to reprise his role as Pied Piper on "The Flash." Entertainment Weekly reports he'll appear in season two's 17th episode, which is titled "Flash Back." Barry will encounter Pied Piper in an earlier timeline, while Piper attacks his former STAR Labs coworkers.
In the season one episode "The Sound and the Fury," Hartley Rathaway -- a.k.a. Pied Piper -- was a former STAR Labs tech who gained mysterious sonic powers after he lost his hearing in the particle accelerator explosion. He crossed paths with the Flash during his quest for revenge against STAR head Dr. Harrison Wells.
As Barry will return to an earlier timeline in "Flash Back," it seems likely he will encounter Pied Piper at this point: at the beginning of the character's villainous career.