The Empire of Evil, by Gordon Strong (paperback)
Magic is a paradox, one that can never be fully resolved. Within other worlds are mysteries beyond the imagination. It is within these extraordinary realms that our psychic investigator willingly explores. Standley-Strange, the arch magus, scholar and man-of-the-world is saved from becoming an eternal recluse by the vivacious Cyndi.
Fleeing to England from a dangerous darkness in America she has sought shelter, her intuition guiding her unerringly to the very door of Standley-Strange! They speedily become magical allies and Standley-Strange later initiates his willing pupil into the Mysteries.
Called to the aid of Debroneth, a Medieval province, inexplicably manifested in another dimension, the magus vows to protect its people from the ravages of the Evil Empire. Standley-Strange must also face his own challenges when he confronts the Emperor Tortius and Bredon Shaft, his vicious Chief Inquisitor.
Treachery, tyranny and a ruthless desire for power are the hallmarks of the Empire.