Celtic Mythology

Celtic Mythology is the mythology of the Celts, a group of tribal societies present across Europe during the Iron Age and in Medieval times.

In Toaru Majutsu no Index, Celtic Mythology is used as a base for certain forms of Magic.

Principles
There are two main (surviving) strands of Celtic mythology: Goidelic (Irish/Scots/Manx) and Brythonic (Welsh/Cornish/Breton). While they share many traits and have certain figures in common, they do not really overlap; each has its own unique stories. They are further split into 'Cycles' (Ireland) and 'Branches' (Wales). Mainland Europe's Celtic traditions were mostly lost due to invasion and assimilation of Celtic populations in their conquerors' own societies (mainly the Roman Empire and Germanic tribes). The cultural taboo against consigning knowledge to writing certainly didn't help.

Background
According to the Celts, before people came to the archipelago we now call the British Isles, a race of intelligent magical non-humans calling themselves the Tuatha Dé Danann ("the children of the goddess Danu") lived there. With the arrival of people and their permanent settlements, the Tuatha Dé Danann continued to muck about in the lives of people, but retreated to the Otherworld, their home world, a world still reachable through places such as fairy forts or fairy burrows. This Otherworld is probably the "Fairy Island" Othinus mentions when explaining the Phases.

Uses of Celtic Mythology in the story

 * Cú Chulainn: The legendary hero is mentioned in the narration when Othinus throws her Gungnir at Kamijou Touma, as apparently having been killed by his own spear as did not return to him unlike the other legendary weapons in mythology.
 * Fragarach: The sword of Manannan mac Lir and Lugh. When held at one's throat, the sword was said to prevent them from telling a lie, hence its name, meaning 'Answerer'.
 * Fragarach was one of the weapons analyzed by the Knight Leader in his development of Hrunting.
 * Selkie: Is a legendary being that takes the form of a woman. It is depicted in William Orwell's escutcheon as one of its charges.
 * Sheela na Gig: Kamijou Touya brought a figure to the beach and showed it to Touma, referring to it as a fertility goddess. It is likely that either the origins of this is from Celtic Mythology or part of an ancient folklorish tradition of the British Isles.