Norse Mythology

Norse Mythology is the mythology of the North Germanic people around the time of the Viking Age(at its height covering most of northern Europe, much of modern Germany and Austria, and parts of the British Isles), continuing through into modern-day Scandinavian folklore, and following the lives of the Norse gods — the Æsir and the Vanir — and the men whose lives they directly affected.

In the Toaru Majutsu no Index, Norse mythology is often used as a base for Magic.

Principles
Like most traditional polytheistic systems, it has no set canon and in some ways resembles a body of customary beliefs more than a set religion. It has been speculated that only chieftains and other wealthy people held faith in the Æsir, while the common farmers believed in land-spirits such as dwarves, trolls and giants.

It is a branch of the Proto-Indo-European mythological tradition, which also spawned the Celtic, Greek, and Vedic pantheons; it's distinguished from those myths, however, by the fact that its gods are not only fallible, but also all mortal. They could, and did, die, as exemplified during Ragnarok, the Norse version of the apocalypse, which ends with most of the gods dead.

Many texts describing Norse beliefs have come down to us, but, aside from a few runic inscriptions and similar fragments, all were written hundreds of years after the turn to Christianity. Consequently it's nearly impossible to tell which stories are christianized, or how much they are, although academic theories abound. Even ignoring this, another problem arises: since Norse myth has no definitive canon, the myths differ considerably from place to place, according to the time they were written and the purpose they were written for.

Toaru Majutsu no Index: Kanzaki SS
During the events of this sidestory, Kanzaki Kaori fought a variety of adversaries who made use of Magic based on Norse mythology, culminating in a battle with the Valkyrie Brunhild Eiktobel who sought to complete Gungnir.