The Calamities movement that approached the city was not a natural reaction, but an action from an unknown group. This group lured the calamities into the city region. The weakened barrier was just an additional push for the calamities to stay.
The group appeared two days after he inherited the Grandmaster Codex. They made no contact with him and had been staying away from him. They were extremely cautious, making sure to keep their distance from Tang Shaoyang. Maybe they tried to use the calamities to test out how strong Tang Shaoyang was.
That was the expected guests in Vandir's words. The reason he called the group of cultivators from the Black Lotus Sect unexpected guests was because of their unknown origin.
"I don't think they will show up after witnessing what I did to these calamities, as well as the group from the Black Lotus Sect."
Tang Shaoyang could not see this unknown group with his Chaos Eyes. They stayed far too far away from his Chaos Eyes' reach.
