In Toaru Majutsu no Index, Nyarlathotep is a being which can be brought into existence through certain spells based off the Cthulhu Mythos.
Like the other evil gods of the Cthulhu Mythos, Nyarlathotep is a being which no human can handle, however like them it gains its role and terror from the story it represents. Spells which use Nyarlathotep cannot ignore this story aspect - as such it's terror only lasts during the 'story' and it can't do anything once that story comes to an end, until the next story begins.
As with any spell based off the Cthulhu Mythos, the summoning of Nyarlathotep is defined as creating something from nothing.
The "Shining Trapezohedron" (輝くトラペゾヘドロン Kagayaku Torapezohedoron?) is a trapezohedron-shaped jewel, based off the jewel found in the story "The Haunter of the Dark". In the story, the jewel was a medium used to summon Nyarlathotep, kept in a special sealed box. The jewel's power was not invoked when the seal on the box was broken, but rather when the lid was shut and the jewel covered in darkness.
In order to use the jewel, the user speaks a chant and then covers the jewel in darkness. Once the jewel is covered in darkness, Nyarlathotep is then summoned as a mass of black smoke and sends lightning in all directions, enough to knock people unconscious. Since the jewel requires darkness, if any light reaches the jewel, the ceremony will end and the smoke will vanish instantly.
The chant required to invoke the jewel's power is as follows:
- "From the moment I become involved with you, I vow that I will not return alive. Oh mocking god, use the darkness filling this stone to destroy everything for this one last night." 
According to the thoughts of Vase, who made use of the trapezohedron, it is apparently possible to use Stonehenge as a box with the trapezohedron to perform a high-quality summoning of Nyarlathotep, powerful enough to easily wipe out the forces around the circle though putting them at risk. Vase chose not to attempt this as other methods were more preferable.
Necessarius Special Admission Test SSEdit
Vase, a Magician of the Dusk Waiting to Awaken, makes use of a trapezohedron to summon Nyarlathotep while fighting against the Amakusas at Stonehenge. During this battle, he covered the trapezohedron by clenching in his fist and then by swallowing it, though he was forced to cough it up after his defeat for his own safety. The trapezohedron was later used by Freadia Strikers to defeat Emilie Fordia by connecting it to her sword and forcing it to use a form so terrible that the trapezohedron's effects would class as healing.
- ↑ Toaru Majutsu no Index SS: Necessarius Special Admission Test Chapter 4
- ↑ Toaru Majutsu no Index SS: Necessarius Special Admission Test Chapter 2
- ↑ 3.0 3.1 3.2 3.3 3.4 Toaru Majutsu no Index SS: Necessarius Special Admission Test Chapter 5
- ↑ Toaru Majutsu no Index SS: Necessarius Special Admission Test Chapter 6