The Missing Child.

The Misty Cloud Sect went into a state of lockdown while the disciples cultivated behind closed doors. The Patriarch activated the sect's defensive formation, which raised a condensed layer of spiritual energy around the mountain peak. 

No one would get in or out of the sect without being noticed. The formation would allow the Patriarch to banish anyone within its range and prevent them from entering if he chose to. It was almost the perfect way to defend the sect. 

The only other option would be allowing the Spirit Beasts below the mountain to roam free and guard specific spots around the mountain, but that wasn't something they could risk. Allowing their enemies to see their last line of defense could be devastating if they chose to attack soon.