"Abyss Guardians & Abyss Walls" 

After all, the abyss seals were also divided into many parts so that once the seal malfunctioned, not all abyss demons could escape from the abyss. 

Only the demons in that zone where the abyss seal was destroyed would have a chance to escape, just like now.

Now that the number of escaping demons did exceed the predicted number, either the abyss guardians in the past few hundred years had not been doing their job well, or there were some other factors that they didn't know. 

The abyss guardians were people who knew the most about the abyss because they would enter the abyss through a special passage once a month. 

The passage was built to be like in a sea world or aquariums where visitors could stay inside a well-protected corridor but they could see the creatures outside of the corridor without being attacked.