Apparently, Disillusioned Adventurers Will Save the World, Vol. 6 (light novel) - Luke Hutton - Bog

Nick/Zem face off against Nick's one-time mentor, Argus, and the Sword of Tasuki deep in the labyrinth. Tiana confronts Bellocchio, her former teacher, barricaded inside Starmine Hall. Karan searches for the Magic Mirror of Marde-the key to turning the tide of battle-filled with a newfound power. As the enemy breaks through Tiana's siege, the Survivors are brought together once more...but will they be able to stop the Sword of Tasuki from completing his true plan of resurrecting the demon god?!