"Tarzan Triumphant" (Tarzan #15) by Edgar Rice Burroughs

Evil men have come to the land of Tarzan of the Apes, looting and killing as they move. But other evil lurks in a small valley of the Ghenzi Mountains, where the last remnants of an ugly, perverted people dwell in what they call their faith-an ancient belief that came from Rome nearly 2,000 years before. Now they are wicked and many of them are insane. And now they have captured Lady Barbara Collis and mean to use her as a human sacrifice-unless Tarzan arrives in time!