Captive Scorpio by Alan Burt Akers

Dray Prescot, Earthman of Kregen, that wonderful world circling the twin suns of Antares, has risen high in the empire of Vallia, but luck cannot always sustain him. When, at last, all the forces opposed to his lands, his princess, his emperor, and to him personally, converge, it was to produce the darkest hour of his long career. For treason struck at the court, while rebel armies marched from the backlands, the war fleets of enemy nations were aloft, and the uncanny wizardry of a master scientist launched a spell of doom against all that Prescot holds dear. With his back to the wall, Dray Prescot faces that time of peril with unflinching will... until the cruelest blow of all is struck: his warrior daughter Dayra rides in the vanguard of his foes!

Captive Scorpio is the seventeenth book in the epic fifty-two book saga of Dray Prescot of Earth and of Kregen by Kenneth Bulmer, writing as Alan Burt Akers. The series continues with Golden Scorpio.