Grimoire

An "original" grimoire (魔道書) refers to a book containing knowledge about magic. They are considered to be powerful tools with esoteric knoweldge and powerful magic spells written in them.

It is harmful to all who dare read its contents, although, currently, only Index is immune to the debihiltating effects of the grimoires, and with her perfect memory, allows her to serve as the vessel of the Index Librorum Prohibitorum. However, suspicions rise that she would rebel and become a Majin. This suspicion marks the catalyst for the events that would lead to the beginning of Toaru Majutsu no Index.

Principles
As stated, original grimoires are harmful as reading its contents would drive a human into insanity, and are even harmful to those that are trained to handle them.

According to Index, what is written inside of a grimoire is of a different world, and contradicts common sense, a knowledge that is far too alien to be safely comprehensible to any normal human. The knowledge that a grimoire possesses is "poisonous" in the sense that it is too "pure" for the human mind to handle and must be "watered down" – i.e. written in such a way that many of its meanings are obscured – in order for people to be able to read it without their minds being destroyed. Once a human is stained with the poison that is the contents of a grimoire, even inquisitors cannot remove the poison from the mind even after five years of baptism.

Types of grimoires
A grimoire doesn't have to be in the shape of a book, for example: Aztec grimoires being written on animal skins and in stone tablets.

Oriana Thompson has also written up a form of disposable grimoires comprising flashcards, which allows her to use many different kinds of magic. Its drawbacks include its inability to cast the same spell twice, so she has to keep writing grimoires in order to move forward; and its instability makes it destroy itself after a few weeks at best, as such it is not exactly a true grimoire.

Abilities and Behavior
Grimoire is practically "eternal" in the sense that it cannot be destroyed with current human methods (such as burning) and can only be sealed away. Original grimoires can apparently convert the words that are written inside their pages into magical code, which can be described as an auto-powered magic array or spell. The grimoires use the small amounts of energy released by nature or lifeforce as well as the weak magical energy released by humans, because of this attribute of acting like an auto-powered magic array, a grimoire can never be truly destroyed.

Index states however, that since she is unable to use her lifeforce to create mana (due to the restrictions on her made by the Necessarius), a grimoire manuscript that she writes down will not create such phenomenon, as such they can be destroyed, when normally a magician and even a normal human who releases magic energy through their lifeforce when writing a magic will have this problem.

The structure of a grimoire is compared to that of a Magic array, with Stiyl demonstrating how a magic circle written with words can end up having 2 or 3 additional lines surrounding the original circle, like sentences on a page.

Sentience
Grimoires are seemingly sentient enough to disclose its contents to people who desire information from it. Grimoires can apparently identify whether someone was a "reader" or "not a reader", and have the tendency to cooperate with those who would propagate the information with their contents.

As such, a magician could trick the decision-making capabilities of the grimoire under the belief that they could act as an evangelist for its knowledge. Regardless of this, the contents of the grimoire is still dangerous to anyone who reads it and those who attempt to take control of the grimoire. It is currently unknown if a grimoire is aware that they are dangerous to people.

If the grimoire judges its current owner to be unsuitable, it will kill him/her and seek out the next one, as shown in Volumes 15, when Etzali needed to protect Xochitl from her own grimoire and in volume 19, when a grimoire actually killed it's wielder as he was inadvertently trying to prevent it's knowledge from being handed down.

Countermeasures
One theoretical way to destroy a grimoire, according to Orsola Aquinas, is to write data that will force it to destroy itself into it. However, it is unknown if someone has successfully managed this, as the writer still needs to see the pages of the grimoire to actually write down the code, and even a mere glimpse of a grimoire is already dangerous. Kamijou Touma also suggested the possibility that his Imagine Breaker would be able to fully destroy grimoires, although he has yet to make any attempt.

Aureolus Izzard's job before he met Index was to write up vaccines that can destroy grimoires for the Roman Catholic Church. It is unknown if the writer of an original grimoire can destroy their own creations.

It should be noted that when a magician has been enough skill, they can read the contents of a Grimoire and not suffer from it's contamination, one of this cases was Aureolus

Politics
Not unlike Saints, grimoires carries a somewhat heavy political pull, with it containing powerful spells as well as being indestructable, because of these, grimoires are sealed in their respective holding places such as libraries and cannot normally be stolen.

Grimoires are also used by the organizations keeping them to promote themselves, as with the case with the Roman Catholic Church displaying the Book of the Law to the public. Furthermore, a lost grimoire could spell turmoil between the magic organizations, as what had occurred with Amakusa Catholics being accused for stealing the Book of the Law, causing the Anglican Church, Roman Catholic Church as well as the Knights of England to take action.

As such, magic societies will not simply leak the contents of the grimoires they keep for no reason, one of those rare reasons were to allow Index of Necessarius to memorize the grimoires for the purpose of reverse-engineering and neutralizing them, and with the 103,000 grimoires inside Index, there is a possibility that all magic in the world can be neutralized. Because of her status as a magic library, Index is reviled by magic societies for knowing the knowledge of the grimoires, knowledge that can be used against those who originally owned the knowledge before Index came along. As such, Index is being hunted by Magic cabals, and is feared by her own church.