Secret Scorpio by Alan Burt Akers

The brightest star in the constellation Scorpio is the brilliant double sun Antares, around which orbits the inhabited planet called Kregen. Kregen is an Earthlike world, but strange, far stranger, than ours. For it is the scene of a conflict between galactic powers who utilize its many human nations and its astonishing variety of humanoid peoples as their battleground. Dray Prescot of Earth had been a tool of those powers, but courage and ingenuity has won him a high role in the Vallian Empire and a certain independence of his own. So when a mysterious monster cult begins to undermine the empire and when his own beloved princess becomes a victim of those secret schemers, Prescot has to go into action.

Secret Scorpio, the fifteenth book in the epic fifty-two book saga of Dray Prescot by Kenneth Bulmer, writing as Alan Burt Akers, launches a new cycle in the magnificent history of Kregen and brings Prescot at long last into direct confrontation with the forces that had once dared make him their pawn. The series continues with Savage Scorpio.