The Misfit of Demon King Academy 04: History's Strongest Demon King Reincarnates and Goes to School with His Descendants (manga)
Anoth has freed Ivis from possession by the false Demon King, and peace has been restored for the time being… But it doesn’t last for long! New transfer student Rei joins the class at Demon King Academy Delsgade and immediately makes a splash with his superior fighting skills. Rei’s swordsmanship may be exceptional, but will it be enough for him to win when he challenges Anoth to a duel? Or is it time for the Demon King of Tyranny to school his fellow student?