Itzcoatl's last codex

Itzcoatl's last codex was an Aztec document started by ruler Itzcoatl in the mid 15th century, and kept secret after all other codex documents were intentionally destroyed. It is not currently understood what it depicts. 

History
At some point after or during the fall of the Aztec Empire in the mid 1400s, the codex was saved from destruction, where others were not, and hidden in an Aztec temple on a mountain in Mexico, protected by traps. In the early 21st century, an earthquake loosened the soil around a hidden entrance to the temple which the locals came aware of. Through word-of-mouth, Jacques learned of this entrance and used his in-field archaeology program, which was digging nearby, to investigate for himself. Jacques found the codex and brought it back to North America, where he donated it to the Trinity Museum of Antiquities. 

Shortly after, the codex was put on display, at the museum, where it was stolen by members of a crime syndicate. Decoding some of its meaning, both Jacques, through photos, and the crime syndicate group, through its stolen acquisition, went to Teotihuacan in Mexico to search for an unknown find described on the codex itself. Encountering Jacques, they revealed they were in possession of the document. 

While visiting Trinity Museum, Reese Hunter mentioned, in passing to Leanna Boswell, that he had studied Itzcoatl's last codex extensively. (: "Modern Abyss")