The Demon Sword Master of Excalibur Academy, Vol. 5 (manga)

After the dust settles from the Hyperion incident, Excalibur Academy receives a strange distress signal—one that appears to be from the Third Assault Garden. The problem? The Third Assault Garden should have been destroyed by Voids six years ago! Determined to find out what happened to her birthplace, Riselia leads the eighteenth platoon on a new mission: exploring the abandoned city. But what awaits them may be more than just ghosts from the past…