High School Prodigies Have It Easy Even in Another World!, Vol. 6 (manga)

Though the high school prodigies' revolution had been going smoothly so far, all that changes when the Blue Brigade conspires with the empire to betray them. Worse still, they soon find that Gustav, presumed dead, is very much still alive and zealously intent on stopping them in their tracks. And for some reason, the warning he gives them haunts Tsukasa's dreams: "Such hubris, for a common impostor. We cannot save humanity."