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. It is a scroll written on animal skin, which describes the times from the calendar dealing with life and death, expanded on with a religious dissertation. It was previously merged with Xochitl, with two-thirds of her body having been ground down and used as the base for the grimoire.

With 'counter-attack against weapon wielders' inscribed on it, the grimoire has a defensive function which forces enemies to kill themselves with their own weapons. While merged with the grimoire, Xochitl made use of this spell by spreading her powdered form of her ground flesh into the air, where it would be carried by the wind onto her opponent's weapons. There was a magical line connecting Xochitl to her severed flesh (connected to how, in Aztec rituals, human flesh is eaten in order to deliver it to heaven), and as such the powder magically qualified as part of her body, allowing her to control it and objects it was packed around just by thinking. By making her enemy's weapons part of physical body, she could control them and force the enemy to commit suicide with their own weapon.

When wielded by Etzali, the scroll moved in the air in response to his will, wrapping around him and expanding in order to intercept projectiles, and being able to withstand the 'rabbit bone' bullets of the Moon Rabbit which were capable of smashing through the walls of a nuclear shelter. It also protected his body, such that several direct hits to the torso from the aforementioned projectiles didn't tear him apart, though he was knocked down and left unable to stand. While Etzali wasn't able to use spells of the suicide spell's magnitude when he fought Tecpatl, the grimoire launched counter-attacks towards the Moon Rabbit's stone tablets by blowing powder stuck onto its skin.

The grimoire also has a spell capable of making artificial body parts, which was used to save Xochitl as her body was collapsing, after Etzali tricked the grimoire's decision-making ability into thinking that it wouldn't be able to pass itself on if she died.

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'.

The grimoire is based on the story of the 's creation, in which the moon that was created at the time was equally bright and a rabbit was thrown at it to weaken its glow. Using this legend, the grimoire can fire 'rabbit bones' as powerful projectiles, capable of smashing through the walls of a nuclear shelter. With 'long distance bombardment' inscribed on it and using the legend of rabbits being thrown at astral bodies, this grimoire's spell can destroy most enemy flying spells and enchantments.

Human bones can be used as raw material for the 'rabbit bones' and the grimoire can use a recorded method to replace the taken bones with obsidian, a process which is apparently excruciatingly painful. When Tecpatl wielded the grimoire, he used the quasi-comatose Tochtli's bones instead of his own.

When Tecpatl used the grimoire's spell, the shots were fired from his hand with a flash of light. He also claimed that it was originally a strike which could reach other astral bodies like in the legend, putting the apparently reduced power down to the 'rabbit bones' he was using being inferior.

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.