Seiðr (which is sometimes anglicized as seidhr, seidh, seidr, seithr, or seith) is an Old Norse term for a type of sorcery which was practiced in Norse society during the Late Scandinavian Iron Age. Connected with Norse paganism, its origins are largely unknown, although it gradually eroded following the Christianization of Scandinavia. Accounts of seiðr later made it into sagas and other literary sources, while further evidence has been unearthed by archaeologists. Various scholars have debated the nature of seiðr, arguing that it was shamanic in context, involving visionary journeys by its practitioners.
Seiðr’s practitioners were of both genders, although females are more widely attested, with such sorceresses being variously known as vǫlur, seiðkonur and vísendakona. There were also accounts of male practitioners, known as seiðmenn, but in practicing magic they brought a social taboo, known as ergi, onto themselves, and were sometimes persecuted as a result. In many cases these magical practitioners would have had assistants to aid them in their rituals.
Within pre-Christian Norse mythology, seiðr was associated with both the god Oðinn, a deity who was simultaneously responsible for war, poetry and sorcery, as well as the goddess Freyja, a member of the Vanir who was believed to have taught the practice to the Æsir. — Wikipedia
Art by Marc Potts
source: The Northern Grove