Private Tutor to the Duke's Daughter: Volume 5 by Riku Nanano

After thwarting a nascent rebellion and restoring peace to the eastern capital, Allen prepares for some long-overdue rest. His students have returned to their respective homes, leaving the convalescent tutor in his family’s care...but the lazy days of summer are only the calm before the storm. Sinister forces are at work in the east, and they threaten to shake the kingdom to its very core. Shadowy conspiracy erupts into open war, separating Allen from his powerful companions and—worse yet—his trusty partner Lydia. The young sorcerer must turn to his fiercely protective sister Caren, his friend Richard, and a small band of knights in a battle for all he holds dear. But how much can so few accomplish against an army of unexpected foes wielding mysterious and devastating magic?