Deadly Villainy in Death Valley

3rd part of Karl May's Colportage Novel "Deutsche Herzen, Deutsche Helden" - Im Tal des Todes (In Death Valley) as a completed novel

Why is Prince Oskar von Steinbach travelling far and wide to find the missing Von Adlerhorst family, and pursuing the villains who spoke the curse? What secret connects his father to the affair? What role does the Prussian Cavalry Captain Guenther von Langendorff play in the unveiling of the mystery?
The murder in Constantinople two decades earlier set the wheels in motion for Steinbach and his friends, Sam Barth, Jim and Tim Snaker, the Apache chief Strong-hand, Langendorff and Zimmermann to ride across the Wild West—from the Mission at Silverlake, to Prescott in Arizona, up the Rio Colorado, across the Mojave Desert, and into Death Valley—and liberate not only two damsels in distress, but also the slaves in a mercury mine in the Valley of Death.
The villains are caught and transported to San Francisco, where Steinbach receives documents sent from his brother in Germany; at last, the revelations in the letters unveil the secrets—the sins of the fathers—of which even the prince was unaware.