Herzog stumbled upon a children's book about adventure in a friend's house. The book mentions Aguirre, a Spanish conqueror who sought Eldorado and declared himself a "Scourge". Although there is not much content about Aguirre in the book, Herzog was fascinated by the story and tried to find more of his life. However, he learned little in the historical documents, but found that many literary works talked about This person. Herzog found a letter supposedly written by Aguirre to the King of Spain. In the letter, Aguirre announced the deposed King of Spain and declared himself to be the new Eldorado and the new ruler of Spain. That crazy language made Herzog very interested. Herzog said that he was attracted by Aguirre because he was the first person who dared to provoke the Spanish king and declare the independence of the South American tribes. He was also absolutely crazy. It was not only the regime but also nature itself that resisted.
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Many characters in the film are fictitious, and some characters have never actually participated in the expedition described in the film even if they exist in history. Herzog made up the character based on some of the names he read in the original documents. The voice-over fictionalizes the diary of a monk Gaspar de Carvajal who participated in the expedition. Herzog claimed that there was indeed such a monk with the same name, but that monk only wrote a diary for another unrelated expedition.
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