Tatsuya Sudou had always been troubled by mysterious voices resounding on his head, threatening to drive him insane. His teacher, Akinari Kashihara, believed the voices belonged to aliens wishing to spread a message, and wrote them down in the single volume that was In Lak'ech. In reality, the voices were all Nyarlathotep, orchestrating the downfall of Humanity.
The major prophecies contained within the volume are the Oracle of Maia poem, and a text which tells of the new advent of the Last Battalion - a mythical final group of Nazi sympathizers returning for another bid for control of the world. Specifically, the book "revealed" the Nazis had been hiding in Antarctica, rebuilding their ranks and growing in power until they were ready to make their bid for global domination.
While nothing more than a simple rumor, the book's release after the Masked Circle had successfully forced the Oracle of Maia's poem prophecy's completion allowed Nyarlathotep to use Sumaru's powers to bring the Last Battalion to reality from nothing in a bid for the power of Xibalba.
In the Eternal Punishment timeline, the book was never written. However, it was found, strewn with other memorabilia of both Innocent Sin and Eternal Punishment in Philemon's Extra Dungeon mode. It was one of the items required to restore Tatsuya Sudou's memory for the next scenario.
- Okamura is largely correct in her translation, with the phrase being a Mayan greeting used by priests. New Age writers such as Domingo Martínez Paredes link it to similar monotheism/unity Judeo-Christian themes.