My Here Academia: School Briefs novel volume 5 features story by Anri Yoshi and is inspired by an idea by Kohei Horikoshi.

It’s time for class 1-A to start getting ready for the end-of-the-year holidays, and that means undertaking a winter cleaning. The unsuspecting students get a big surprise when their cleaning leads to a crazy discovery. What’s this mysterious entrance?! They have no choice but to rustle up some Plus Ultra spirit and venture inside!