Headcannon. The apprentice told Zoso he had no idea what he’d unleash if he usef the hat. I think the hat was used to imprison the first Dark One: Chernabog. As the most powerful being it took a lot of magic to subdue him and would take a lot to unleash him. See, Rumpel thinks when he gathers enough magic, he can use it to be unstoppable and be free from the dagger, but really when the hat has absorbed enough power Chernabog will be strong enough to be set free. SOMETHING has to be inside that hat that is so bad it needs such protection. The apprentice said speicifally something would be unleashed. Something or someone is in that hat. ..
I was totally think the first thing since I saw the hat…I was like, I wonder if Chernabog’s in there…I really hope so.