Seidr (pronounced “SAY-der;” Old Norse seiðr, “cord, string, snare”) is a form of pre-Christian Norse magic andshamanism concerned with discerning and altering the course of destiny by re-weaving part of destiny’s web. To do this, the practitioner, with ritual distaff in hand, enters a trance (which could be accomplished through numerous means) and travels in spirit throughout the Nine Worlds accomplishing his or her intended task. This generally takes the form of a prophecy, a blessing, or a curse. Archaeologist Neil Price has provided an excellent summary of the known uses of seidr:
There were seiðr rituals for divination and clairvoyance; for seeking out the hidden, both in the secrets of the mind and in physical locations; for healing the sick; for bringing good luck; for controlling the weather; for calling game animals and fish. Importantly, it could also be used for the opposite of these things – to curse an individual or an enterprise; to blight the land and make it barren; to induce illness; to tell false futures and thus to set their recipients on a road to disaster; to injure, maim and kill, in domestic disputes and especially in battle.
The Norns are the foremost masters of seidr. However much destiny may be altered by gods, humans, and other beings, its initial framework is established by the Norns. To do this, they use the same means as any norn (Old Norse for “witch”) with a lowercase “n:” weaving, carving runes, and other mainstays of the toolkit of pre-Christian Germanic magic.
For more information please view the sourced page; http://norse-mythology.org/concepts/seidr/