The Noiseblaster is a dangerous weapon created by Noisemaster, which was unsuccessfully used against the Melody Kingdom. It has the power to convert screams into destructive energy - fully charged, it would have had enough power to eradicate all of Trebleopolis.
In Chapter 2, Princess Piano was captured by Noisemaster to power the Noiseblaster with her screams for help. However, Piano figured out the intention of the Noiseblaster and stopped screaming, meaning that it charged significantly slower. Our Heroes became distracted with battling Noisemaster, and didn't notice that the Noiseblaster reached 100%. Luckily for them, directly after the Noiseblaster fired it was stopped by a mysterious, purple, starry shield.
In Chapter 3 it has been mentioned by Glitchmaster, who said that the Noiseblaster was fully functional, but was sabotaged at the last minute by the Nightmare Knight. It is unknown whether or not this is true, or if Glitchmaster is manipulating Rosemaster.