Witch Swap Appearances

It is now told that Signý was sitting in her room one time when a very
powerful witch visited her. Signý said to her: “I would like for the two
of us to switch our forms.”

The witch answered, “That’s for you to decide.” And so the witch
used her magic to cause the two women to exchange shapes. The witch
sat in Signý’s room in place of Signý, as Signý requested, and in the
evening she went to bed next to King Siggeir. And the king did not
discover that it was not Signý next to him.

Now Signý went to where her brother lived in the turf-house, and
she asked for lodging for the night “because I’m lost in the forest, and I
don’t know where I am.”

He said that she could stay there, as he wouldn’t deny hospitality to a
woman traveling alone, and he didn’t expect her to repay his generosity
with betrayal. So she came into the house and they sat down and ate.
Sigmund’s eyes were often drawn to the woman, and he thought she
was truly beautiful. And when they had finished eating, he told her that
he wanted them to share one bed that night. She said nothing against
this, and Sigmund laid her down on his bed three nights in a row.
After this, Signý went home and met the witch again. She asked that
they exchange looks once again, and they did so.

And after some time passed, Signý gave birth to a boy, and he was
given the name Sinfjotli. And as Sinfjotli grew up, he proved to be big
and strong and good-looking, and very much of the Volsung type.
He was not yet ten years old when Signý sent him to Sigmund in his
turf-house.


- Volsunga Saga, Chapter 7


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Note: In the original Old Norse, the powerful witch in this passage is explicitly a seiðr user. Here's an excerpt:

"Þess er nú við getit eitthvert sinn, þá er Signý sat í skemmu sinni, at þar kom til hennar ein seiðkona fjölkunnig harla mjök. Þá talar Signý við hana: "Þat vilda ek," segir hún, "at vit skiptum hömum." Hún segir seiðkonan: "Þú skalt fyrir ráða." Ok nú gerir hún svá af sínum brögðum, at þær skipta litum, ok sezt seiðkonan nú í rúm Signýjar at ráði hennar ok ferr í rekkju hjá konungi um kveldit, ok ekki finnr hann, at eigi sé Signý hjá honum. Nú er þat frá Signýju at segja, at hún ferr til jarðhúss bróður síns ok biðr hann veita sér herbergi um nóttina, "því at ek hefi villzt á skóginum úti, ok veit ek eigi, hvar ek fer." Hann mælti, at hún skyldi þar vera, ok vildi eigi synja henni vistar, einni konu, ok þóttist vita, at eigi mundi hún svá launa honum góðan beina at segja til hans. Nú ferr hún í herbergi til hans, ok setjast til matar. Honum varð opt litit til hennar ok lízt konan væn ok fríð. En er þau eru mett, þá segir hann henni, at hann vill, at þau hafi eina rekkju um nóttina, en hún brýzt ekki við því, ok leggr hann hana hjá sér þrjár nætr samt. Eptir þat ferr hún heim ok hittir seiðkonuna ok bað, at þær skipti aptr litum, ok svá gerir hún."

Taken from the Old Norse version of Volsunga Saga available at heimskringla.no.