Reign of the Seven Spellblades, Vol. 5 (manga)

As the battle to determine the strongest first-year heats up, Katie comes to her friends with a daring proposal—to build a secret base within the labyrinth! Having a shared workshop would be a great boon for each of their personal projects, but the dark corridors under the school are a perfect spot for an ambush. With new opponents entering the fray nursing grudges against the group, Oliver and the others will have their work cut out for them if they want to stake their claim!