Grimoire

An "original" grimoire (魔道書) is a book containing knowledge about magic that is harmful even to those that are trained to handle them. The knowledge 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. Even then, it can still severely damage a person's mind if not properly handled. Grimoires often contain powerful spells or magic and can be very dangerous. Necessarius was set up to prevent grimoires from spreading in the first place, along with the goal of eliminating witches.

A grimoire is practically "eternal" in the sense that it cannot be destroyed with current human methods and can only be sealed away. It absorbs natural mana from the Earth itself, in addition to the minuscule mana magicians release unconsciously, to keep functioning. The structure of a grimoire is compared to that of a magic circle, 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. The only theoretically known way to destroy a grimoire, according to Ursula Aquinas, is to write data that will force it to destroy itself into it, though Touma has also considered the possibility that his Imagine Breaker can destroy grimoires. Aureolus's job before he met Index was to write up vaccines that can destroy grimoires for the Roman Catholic Church. A grimoire also has a "mind" of its own, seeking out suitable owners that can spread its knowledge and improving itself by letting its owners add more knowledge into the grimoire. Touma, before losing his memories, once described a grimoire as a computer virus that's continously evolving and spreading itself at a rate faster than that of its vaccine being designed. 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 and 19.

Index currently has 103,000 grimoires stored within her brain while Etzali has obtained two Aztec grimoires after defeating their original owners. Yamisaka Ouma captures Index to seek out one of her grimoires, in order to save a woman he loves.

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