The Gun has the ability to turn people back into babies. The effects are permanent to a point. Someone that was turned into a baby can continue to grow up again, Professor Medulla was later able to scientifically reverse the process.
Royal Pain invented the weapon to turn heroes into babies so she could raise them as villains. She was defeated by The Commander and Jetstream. During the battle, The Commander damaged the gun and because of it, Royal Pain was turned back into a baby herself. The Commander then takes the Pacifier as a trophy and stores it in his secret lair.
Role in the film
Sixteen years later, Royal Pain plots to steal back the Pacifier from The Commander, using his son Will Stronghold. When she gets the weapon back, she again uses it to try to turn the heroes into babies. She manages to turn most of Sky High School's staff and students into babies, before she was defeated by Will. Mr. Medula was able to reverse the Pacifer's powers and then destroys it afterwards so it can never be used again.