Necronomicon

The Necronomicon (死霊術書(ネクロノミコン)) is a grimoire found in Toaru Majutsu no Index. Originally a fictional grimoire introduced by to enhance his stories in his Cthulhu Mythos stories, it became a genuine grimoire after real magicians became enamored with the Cthulhu Mythos and applied real magic techniques to make it a reality.

The Necronomicon, originally being a fictional grimoire in the Mythos that anyone can read, has now become a real original grimoire, poisonous to the mind to those who read it.

Etymology
Lovecraft invented the word Necronomicon. According to him, the word Necronomicon, as translated from the Greek language, meant "an image of the law of the dead": nekros - νεκρός ("dead"), nomos - νόμος ("law"), eikon - εικών ("image").

Similarly, he made up the word Al Azif, and defines it as an Arabic word that means "that nocturnal sound (made by insects) supposed to be the howling of demons."

Appearance
The book is bound by leather from unknown animal's skin, though presumably human as it was the famous symbol of the books and documents in the Cthulhu Mythos. The word Necronomicon is written on the cover.

Principles
In Lovecraft's fictional account on the grimoire, it was originally called Al Azif (アルアジフ).

The "real" grimoire is based off fiction, it is a grimoire of a brand new system that was created by thoroughly coating it with real theories and formulas. It is unknown how the Necronomicon became real. In the narrative it is hypothesized that either a magician became amused at the stories of a Cthulhu Mythos enough to make it real or a magician truly wished to see the world described in the stories with his own eyes. Regardless, someone had searched through the different magic systems throughout the world and perfectly calculated out what was needed to recreate the despair-inducing events written of in the Cthulhu Mythos. This person had written a book filled with the spells that gave free control over that and secretly sent that book out into the world. Afterwards, the grimoire was secretly sent the book out into the world, and somehow found itself in the national library of London.

The "real" grimoire however, has nothing to do with the fictional grimoire found in the Cthulhu Mythos. Here, it only told one what was necessary to recreate every one of the hopeless situations featured in the Cthulhu Mythos. It also contained knowledge from other books featured in the Mythos, as well as the "new horrors" invented by later authors to add to the Mythos after Lovecraft's death.

As it is an original grimoire, reading the contents of the book poisons the mind of the reader enough for them to collapse, as shown by Arlands Darkstreet's minions as they analyzed the Necronomicon.

Background
After being secretly sent out into the world by the author or authors, the Necronomicon found itself in the national library of London. Index later visits the library and read the contents of its pages as one of the 103,000 grimoires she memorized.

Toaru Majutsu no Index
It is mentioned by Index in her explanation on grimores with The Book of Eibon, the Lemegeton, Unaussprechlichen Kulten, Cultes des Goules, and the, but comments that because it is so famous there are all sorts of imitations and fakes making it unreliable.

Toaru Majutsu no Index SS: Necessarius Special Admission Test
The original grimoire is temporarily stolen by Flack Anchors of the Dusk Waiting to Awaken who uses his gun magic to shoot out sounds based off the chosen pages of the Necronomicon to Arlands Darkstreet, leader of the magic cabal, who then uses several skilled magicians to analyze the sounds and write it down again as a new copy of the Necronomicon.

The original grimoire is returned to the national library of London by the Amakusa-Style Remix of Church.