Heimskringla Seiðr

All these skills he taught along with runes and those songs that are called galdrar (‘magic spells’).
Because of this the Æsir are called galdrasmiðir (‘magic makers’). Óðinn
knew, and practised himself, the art which is accompanied by greatest power,
called seiðr (‘black magic’), and from it he could predict the fates of men
and things that had not yet happened, and also cause men death or disaster
or disease, and also take wit or strength from some and give it to others. But
this magic, when it is practised, is accompanied by such great perversion
that it was not considered without shame for a man to perform it, and the
skill was taught to the goddesses.


- Heimskringla, Ynglinga Saga, Chapter 7