The Hidden Dungeon Only I Can Enter (Manga) Vol. 6

At the invitation of his brother, who's studying abroad, Noir heads off to the town of Honest with Emma and Lola. Instead of a relaxing vacation spot, they find a city being attacked by a swarm of monsters! They learn that this "Great Invasion" occurs regularly, and to his surprise, Noir discovers that the person behind this event is someone he knows very well.