Tarzan and the Forbidden City(1938)

Edgar Rice Burroughs

Tarzan cared little for the fate of adventurer Brian Gregory, drawn to the legendary city of Ashair by the rumor of the Father of Diamonds, the world's hugest gem. But to the ape-man the tie of friendship was unbreakable, and Paul d'Arnot's pleas moved him to agree to guide the expedition Gregory's father and sister organized for his rescue. The enigmatic Atan Thome was also obsessed with the Father of Diamonds, and planted agents in the Gregory safari to spy out its route and sabotage its efforts. Both parties reached their goal, remote Ashair . . . as prisoners of its priests, doomed to die in loathsome rites.