Sword Art Online, light novel Vol. 11: Alicization Turning

Two years into Kirito and Eugeo's quest to reach the Central Cathedral, the pair have finally become elite disciples at the North Centoria Imperial Swordcraft Academy. Now all that's left to do is train for the next tournament, build their relationships with their trainee pages, and do so without violating the Taboo Index. But just because this peaceful world is governed by law and order doesn't mean evil can't fester below the surface...and when it comes time to choose between the rules and what's right, Kirito and Eugeo discover the darker secrets of the Underworld.