The Aztec Calendar Stone consists of two Aztec Grimoires ("Times related to Life and Death" and the "Moon Rabbit") that are both currently in Etzali's possession, after defeating their owners in Volumes 15 and 19 and taking each off them.
The special system that rules the function of those Grimoires is known as 暦石, literally read as 'koyomi ishi' or calendar stone, Tecpatl discovered means to reproduce the pages of the grimoires and a defensive system that enables the usage of the books without actually reading them; thus preventing the mind contamination that comes from coming into contact with the books. The calendar stone was an Aztec calendar arranged in a circle. However, the Aztecs used two different forms of calendars at once and believed in the death and rebirth of the sun, so it was an incredibly complex thing to make. What was described on the inside of Xochitl’s skin took only the times that dealt with life and death from that calendar and then expanded on it with a religious dissertation.
Time related to Life and Death (生と死に関する時間 Sei to Shi ni Kansuru Jikan??): Has the combat function of countering against any enemy that holds a weapon, having them kill themselves. After her failure in USA Xochitl had a Grimoire based on a derivative of the Aztec Calendar Stone (the Calendar Stone holds the complex calendar the Aztecs use, and also records the process of how the world would destroy itself and its following rebirth). The Grimoire works by grinding down her own body into dust, and as the dust is carried by the wind onto the opponent's weapons the weapons would be seen as part of her body to be controlled and forces the enemy to commit suicide with the weapon. The parts of the body that is used up will be replaced by obsidian, and magic is the only thing anchoring the user's mind and soul to the body. It also has a very powerful defensive function, as demonstrated when it enhanced his body to withstand the bullet from "Moon Rabbit". The Aztecs used two different forms of calendars at once and believed in the death and rebirth of the sun, this Grimoire is born from that concept.
Moon Rabbit (月 の ウサギ Tsuki no Usagi??, lit. "Rabbit of the Moon"): Has the combat function of sniping. Uses the bones of rabbits to shoot out powerful bullets. The bones of the rabbit can be substituted by human bones, which can be purchased by obsidian and are strong enough to penetrate Shiokishi's shelter designed to protect against nuclear weapons. This Grimoire uses the legend of throwing rabbits at astral bodies as it's basis. Tecpatl manged to avoid the price of using the Grimoire by using the bones of a quasi-comatose Tochtli who was being used as "materials" for Tecpatl's attacks.