Castalian, a high-ranking mage, worked incognito as the array maintainer at St. Peter's Academy, solely to prevent the abyss lord from breaking its seal. However, the newly appointed headmistress, believing he was wasting resources and neglecting his duties, dismissed him and replaced him with a dragon knight to guard the academy.
When the dragons attacked, the highly anticipated dragon knight couldn't even withstand a casual blow from the dragon king, forcing him to forcibly activate the array left by Castalian to repel the enemy. This didn't make the headmistress understand who the true protector of the academy was; instead, it made her trust the dragon knight, who was all talk and no action.
When the dragon king realized the array had failed and led his dragons in attack, and the abyss lord broke its seal, corrupting and eroding the academy's faculty and students, everyone finally realized that the one who had protected St. Peter's Academy for millennia was none other than the array maintainer they all despised, Castalian!
