Aztec Calendar Stone

The Aztec Calendar Stone (暦石) is an ancient and complex calendar used by the Aztecs, arranged in a circle and inscribed on stone. Several original grimoires have been derived from the Calendar Stone and used by members of the Return of the Winged One. The two which have been seen so far ("Time related to Life and Death" and the "Moon Rabbit") are both currently in Etzali's possession, after defeating their owners in and taking each off them.

Principles
The calendar stone is a gigantic round stone which records the calendar system used by the Aztecs, as well as the process of the world's destruction and recreation with the, inscribed in a circular arrangement. The fact that the Aztec system uses two   of calendar simultaneously, coupled with the additional solar information, makes the calendar stone an incredibly complex thing to construct.

It is possible to create original grimoires derived from the contents of the calendar stone (usually a specific portion rather than the whole lot due to its complexity) and the magic cabal Return of the Winged One produced such grimoires to arm its members, as part of a plan by Tecpatl, who also devised a means to use the books' spells and defensive systems without reading them and to prevent the flow of knowledge into the user's mind to protect them from contamination.

Time related to Life and Death
Time related to Life and Death (生と死に関する時間) is the first of the known original grimoires derived from the calendar stone.

Has the combat function of countering against any enemy that holds a weapon, having them kill themselves. 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.

It also has a very powerful defensive function, as demonstrated when it enhanced Etzali's 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
The Moon Rabbit (月 の ウサギ) is the second of the known original grimoires derived from the calendar stone. It consists of several stone tablets a few millimeters thick, with Aztec writing and symbols densely carved on them, contained in a rectangular 'bag'.

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.

Background
At some point in the past, Tecpatl, in charge of military affairs and battle operations for the Return of the Winged One devised and later implemented a plan to produce original grimoires derived from the calendar stone and arm the cabal's soldiers with them.

Battle Royale Arc
During Etzali's battle with Xochitl at the Reformatory on October 9th, the latter made use of a calendar stone-derived grimoire, Time related to Life and Death, which her flesh was ground down and used as the base material for, as punishment for disobeying orders during the Liberal Arts City incident. As Xochitl's body reached its limits and start to come apart, Etzali recognized the grimoire's origins after catching a glimpse of it and took possession of it in order to save her life.

DRAGON Arc
When confronting Etzali at Shiokishi's base, Tecpatl made use of another calendar stone-derived original grimoire, this one based on the Moon Rabbit, which fired projectiles which could smash through the nuclear shelter's walls. As well as shielding himself from the grimoire's dangerous knowledge, Tecpatl used Tochtli to supply the 'rabbit bones', with obsidian being painfully inserted to replace the expended bones.

Etzali was enraged by Tochtli's treatment and unleashed his own grimoire to fight against Tecpatl. The two used the calendar stone grimoires against each other, though Etzali experienced greater contamination from the flow of knowledge while Tecpatl had taken steps to not come into contact with it. Etzali was eventually knocked down but managed to exploit the tendency of original grimoires to side with those who could spread their knowledge the most by writing down some of its content in his own blood as Tecpatl was attacking, illustrating he was better suited than Tecpatl and causing the grimoire to turn its last shot back on its wielder, killing him. Etzali then accepted the second grimoire in order to save Tochtli.