Celtic Mythology

Celtic Mythology (ケルト神話) is the mythology of the Celts, an ancient Indo-European group of tribal societies present across the length of Europe from Britain to Asia Minor during the Iron Age.

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

Principles
The Celts worshipped a variety of gods, who appeared in their tales. Most were all-powerful local deities linked to places rather than to specialized roles, with each tribe having its own god who protected and provided for the welfare of that tribe. Some of them had similar characteristics: for example,, the god of life and death in Ireland—known as "the good god"—resembled , the "master" god of Gaul. Some deities also had more clearly defined roles, like Lug, a sun god associated with the arts, war, and healing, and the horned god Cernunnos, who was a god of animals and fertility.

According to the Celts, before people came to the archipelago now called the British Isles, a race of intelligent magical non-humans calling themselves the  ("the children of the goddess " in Gaelic) 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, their home world, a world still reachable through places such as fairy forts or fairy burrows called sidhe. This Otherworld is probably the "Fairy Island" (妖精の島) Othinus mentions when explaining the Phases.

In the Irish myths, as in those of Wales, the heroes often are half-human and half-divine and may have magical powers, like Cú Chulainn, the warrior and champion of Ireland, and the heroic Finn Mac Cumhail, leader of a band of bold warriors known as the Fianna.

Many of the stories in the Mabinogion, a collection of eleven tales of Welsh mythology, deal with Arthurian legends. In fact, the popular Arthurian tales of medieval European literature are a complex blend of ancient Celtic myths, later stories, and historical events. The legends are rooted in Celtic mythology, and references to Arthur appear in a number of ancient Welsh poems.

Background
There were in the past six Celtic nations: Éire, Cymru, Alba, Kernow, Breizh, and Ellan Vannin. (,, Scotland, , , and the respectively.) All Celtic nations had varied mythology and culture, but mainland Europe's Celtic traditions were mostly lost due to invasion and assimilation of Celtic populations in their conquerors' own societies, as well as a cultural taboo against consigning knowledge to writing. Some of our knowledge also come from Roman sources, but, as the Romans sometimes referred to Celtic gods by Roman names and because the Romans and Celts were battlefield enemies, Roman descriptions of Celtic beliefs are often unfavorable and not always reliable. Much of what is now known about Celtic mythology is based on medieval manuscripts, the efforts of later Irish and Welsh monks who wrote down the stories in the 700s and the 1300s respectively.

There are thus two main (surviving) strands of Celtic mythology: (Irish/Scots/Manx) and  (Welsh/Cornish/Breton). They are further split into  (Ireland) and  (Wales). While they share many traits and have certain figures in common, they do not really overlap; each has its own unique stories.

British Royal Family Arc
During the British Halloween, a spiritual item based on Brionac is used by the Knights of England. The Knight Leader also made use of Hrunting, which incorporated analysis of certain Celtic weapons such as Brionac and Fragarach into its development.

Salome Arc
Salome's main spell is External Offering, Celtic-based sacrifice magic which allows her to destroy and offer up weapons to a god, in her case the sea god Manannán mac Lir, in order to increase her strength and gain the destroyed weapons' traits.