Pain
Hao Xuan walked out of the leftmost hallway and made his way through the middle one this time.
The hallway continued for well over five minutes until a vault door came into view, blocking his path forward. The door's material seemed to be a black metal with strange carvings and designs on it.
Hao Xuan moved his hand over the metal and tried to push it open, but it didn't budge at all.
He started putting in more and more force until finally, circulating the Force Qi, he punched out.
The metal door glistened for an instant before returning to its original state. There was not a single scratch anywhere, nor any signs of wear and tear. It still looked as good as new, making Hao Xuan return dejectedly.
He called for Ensis who was still in the library for some reason and ordered him to cut it open. After half an hour of repeated tries, they both walked out tired.
Whatever material the vault door was made of was way too tough for them.