Francisco Diaz was the sole survivor of Cortes' expedition to Tenochtitlán. He lost his sanity which may have contributed to his survival because the Aztecs believed he was touched by the Gods. He lived in Tenochtitlán for 23 years and imparted his Western knowledge to the Aztec priests. Due to miscommunication and to Diaz's mental state, much of the information is erroneous and bizarre. The priests carefully created several codices containing this knowledge.
In addition Diaz explained the system of Spanish writing to Falling Rabbit who later invented a westernized Aztec alphabet. The appearance of wheeled carts in the city are also attributed to Diaz's influence.
The Codex of Iron: Contains descriptions and illustration explaining how iron can be worked and several tools which were common in the western world but were unknown to the Aztecs. It does not have any information about how iron can be mined or refined but is fairly good at explaining how existing iron can be formed. Many of the tools described are fanciful and unworkable but several important basic tools are touched upon.
The Codex of War: The most accurate and useful of the Diaz's codices, the Codex of War describes Spanish military tactics, weapons, and organization.
The Codex of Beasts: An attempt to describe western animals along with how they should be treated and used. Although quite beautiful, this codex is too fanciful to be of much real use.
The Jesus Codex (The Aztec Bible): This disturbing codex present a distorted view of the most gruesome aspects of Christian faith. The Cult of Jesus Christzalcoatl holds this text sacred.