Cthulhu Mythos

The Cthulhu Mythos (クトゥルフ神話) is the name given to the fictional universe, featured in the world of Toaru Majutsu no Index, in which the stories of H.P. Lovecraft (referred to as the "genius author" in the narrative) and other authors after his death are based. It has been used as a base for spells created by certain magicians who desired to see the world described in those stories.

Principles
Since the Cthulhu Mythos is a fictional universe, spells involving it are defined as creating something from nothing.

Many stories in the Cthulhu Mythos describe the rampages of evil gods (such as Cthulhu, Yog-Sothoth and Nyarlathotep) from the perspective of humans on the sidelines - in a sense, they are a catalogue of those evil gods. These evil gods are beings no human can handle, however they all have a point in common - they all gain their role and terror from the story they represent. Ceremonies and spells using these evil gods cannot ignore this story aspect and no matter how frightening, they cannot do anything once the story has come to an end, until the next story begins.

One prominent symbol of the Cthulhu Mythos, used in certain spells derived from it, is human flesh and blood.

Background
At an unknown point after the first stories of the Cthulhu Mythos were published, magicians calculated how to recreate the despair-inducing events in them and created spells with that purpose. They compiled such spells in the Necronomicon, which was then released into the world.

A number of incidents relating the Cthulhu Mythos are reported to have occurred. These include a man attempting to use a jewel in a box to call in someone not of this world, a marriage scam artist sending someone a yellow medallion to have them meet an evil god and steal their inheritance and an old man trying to create an existence guarding a gate that would allow him to travel to another world.

One notable event occurred in the Pacific, three years prior to the current year on August 2nd, orchestrated by the magic cabal Dusk Waiting to Awaken. A ceremony attempting to summon R'lyeh caused 103 workers aboard the resource extraction ship Condor to go insane. However the ceremony, which would have wiped out the residents of every island within 1000 km had it succeeded, ultimately failed. The incident was initially thought to be sabotage aimed at causing diplomatic tension between England and America as well as the international competition over underwater resources. However after the discovery of magical symbols on the ship, England covered up the incident as being caused by rough weather and no land in sight pushing the sailors to their psychological limits, and installed a facility in the area, supposedly to calm the minds of those at sea but actually to keep an eye on where the ceremony took place.

Necessarius Special Admission Test SS
Spells derived from the Cthulhu Mythos are the focus of the events that occur during the Amakusa's admission test for Necessarius. Magicians from Dusk Waiting to Awaken impersonate them in order to steal the Necronomicon.

Using the stolen Necronomicon and a barge in the Strait of Dover, the cabal's leader Arlands Darkstreet begins a ritual to summon R'lyeh. Though this is thwarted by the Amakusa, he succeeds in obtaining data which is transmitted to another group stationed at Stonehenge, who attempt to create the new spell Blank Paper. The leader of the Stonehenge group, Vase, then utilises Cthulhu Mythos-based magic while trying to fight off the Amakusa, before they were all suddenly dispatched by a surprise attack by Freadia Strikers.

Stories
All stories and creatures written by H. P. Lovecraft, except where noted.
 * "The Call of Cthulhu": Arlands Darkstreet and the Dusk Waiting to Awaken try to recreate the events that transpire in the story in order to attain "inspiration" like the poets and artists in the story, as a means of breaking through a dead-end in their magic research by raising R'lyeh twice:
 * The first was on August 2 three years prior to the start of the series. Like in the story, it was in the Pacific. Using the resource extraction ship Condor as a ceremonial temple, it cut off the closed environment of the ship as miniature world and let special rules (the Cthulhu Mythos) eat into the world. The result was that 103 workers went insane and have apparently killed each other based on some common sense of value.
 * The second attempt was on the Strait of Dover, using a ocean salvaging terminal for sunken ships and using float explosions to mimic the earthquakes that heralded the arrival of R'lyeh in the original story as well as using strong lights to mimic the non-euclidean geometry of the city, turning the salvaging terminal into R'lyeh itself. Following the story, a part of Cthulhu himself momentarily appears. Despite the Amakusa Christians foiling their plans, the ritual was successful, allowing the magic cabal to use the "inspiration" brought about by the ritual for their use in creating the Blank Paper spell.
 * "The Haunter of the Dark":
 * In one of the incidents related to the Cthulhu Mythos that Itsuwa reads, one of them mentions a man attempted using a jewel in a box to call in someone not of the world.
 * Vase uses a trapezohedron-shaped jewel with him to represent the jewel found in the story. After a performing a few steps, Vase can cover his jewel in darkness, allowing for a mass of black smoke and lightning shoots out in every direction.
 * "The Hounds of Tindalos" by Frank Belknap Long: A magician uses a pocket watch with an irregularly moving second hand to reflect light. When it hits a target a monster that looks less like a hound and more like a bear trap with four animal legs leaped towards the target. Its appearance in the Toaru Majutsu no Index sidestory where it appeared from a crack in the stones of the plain of Stonehenge and its general description in the narration is a likely reference to this creature.
 * "The King in Yellow" by Robert W. Chambers''':
 * In one of the incidents related to the Cthulhu Mythos that Itsuwa reads, one of them reported on how an enchanting female marriage scam artist used a yellow medallion, the Yellow Sign from the story, to the person she wanted to die, which would cause the person to meet an evil god, Hastur, and would allow her to steal their magical inheritance.
 * A magician uses a yellow medallion as a projectile, and when it hits the target, a being with loose bluish-white skin and wearing yellow rags, representing Hastur, and would bear its fangs towards the target.
 * "The Seven Geases" by Clark Ashton Smith: A magician uses a wire stronger than a violin string that was made by sewing together spider silk to attack a target. When it hits a target, a black shadow that resembled a giant spider appears before it. This is likely a reference to Atlach-Nacha, a spider-like Great Old One from the story.
 * "The Thing on the Roof" by Robert E. Howard: Vase uses a red-jewel which if it hits a target will then emit a pitch black darkness, which then absorbs the target completely even their screams. The jewel represents the red jewel that is said to call in a frog-like evil god found in some story in the Cthulhu Mythos.
 * "The Whisperer in Darkness": A magician covers his skin in mold, and when a target touches the mold the magician makes a chant, at which point the mold becomes tentacles to attack the target. They are based off the Mi-go in the story, an alien race that is also known as the Fungi from Yuggoth. The magician uses them as a motif to strengthen himself in battle.
 * "Through the Gates of the Silver Key": In one of the incidents related to the Cthulhu Mythos that Itsuwa reads, one of them has an old man who tried to "create" an existence that was both guardian of the gate and the gate itself so he could travel to another planet. It is an obvious reference to the story where the aged protagonist meets an Entity, likely Yog-Sothoth who is described as the Gate and the Key, and requests him to take him to an alien planet.

Other references to H. P. Lovecraft

 * Azathoth
 * Dagon
 * Necronomicon
 * Nyarlathotep
 * Shub-Niggurath
 * Yog-Sothoth

References to other authors

 * Hastur by Robert W. Chambers
 * Ithaqua by Algernon Blackwood and August Derleth
 * "The Moon-Lens" by Ramsey Campbell
 * Tsathoggua by Clark Ashton Smith
 * Ubbo-Sathla by Clark Ashton Smith
 * Vulthoom by Clark Ashton Smith