General Seiðr Powers

Men have composed many tales for entertainment, some according to ancient lays or the knowledge of learned men, and sometimes according to old books, which originally were set down briefly, but were later filled out with words, because most events are quicker in the telling. Men are never equally well informed, because it often happens that what’s seen and heard by one isn’t seen and heard by another, even though they’re present at the same event. And it’s also in the nature of many foolish men to believe only what they see with their own eyes or hear with their own ears. They find that what came of the schemes of wise men, or the mighty strength or surpassing skill of great men, is far beyond their own natures—and this is no less the case concerning trickery, or wizardry and mighty magic, when they conjured up eternal misfortune or loss of life for some, and for others, worldly reputation, riches, and honors. Sometimes they stirred up the elements and sometimes calmed them down—men such as Odin was, and the others who learned spellcraft or healing from him. There are even cases where certain bodies have been able to move through the inspiration of an unclean spirit, such as Eyvind Split-Cheek in the saga of Olaf Tryggvason, or Einar Cormorant, or Frey whom Gunnar Half-and Half killed in Sweden.


- The Saga of Hrolf the Walker (Göngu-Hrólfs saga), Foreword


----------

The Old Norse version is referring to seiðr. Here's an excerpt, taken from the Old Norse version of Göngu-Hrólfs Saga available at heimskringla.no.:

"Er þat ok margra heimskra manna náttúra, at þeir trúa því einu, er þeir sjá sínum augum eða heyra sínum eyrum, er þeim þykkir fjarlægt sinni náttúru, svá sem orðit hefir um vitra manna ráðagerðir eða mikit afl eða frábæran léttleika fyrirmanna, svá ok eigi síðr um konstir eða huklaraskap ok mikla fjölkynngi, þá þeir seiddu at sumum mönnum ævinliga ógæfu eða aldrtila, en sumum veraldar virðing, fjár ok metnaðar. Þeir æstu stundum höfuðskepnur, en stundum kyrrðu, svá sem var Óðinn eða aðrir þeir, er af honum námu galdrlistir eða lækningar."

An alternate translation of the relevant section is included in the 2002 dissertation The Viking Way: Religion and War in Late Iron Age Scandinavia, by Neil S. Price. Here's an excerpt:

"Moreover there are plenty of people so foolish that they believe nothing but what they have seen with
their own eyes or heard with their own ears - never anything unfamiliar to them, such as the counsels
of the wise, or the strength and amazing skills of the great heroes, or the way in which seidr, skills of
the mind [huklaraskap] and powerful sorcery [fjǫlkingi] may seid death or a lifetime of misery for
some, or bestow worldly honours, riches and rank on others. These [men] would sometimes stir up the
elements, and sometimes calm them down, just like Odinn and all those who learnt from him the skills
of galdr and healing."