The Svefnthorn (Old Norse svefnþorn, “sleep thorn,” pronounced “SVEFN-thorn”) is a symbol that features in several of the Norse sagas and in folkloric magical formulas recorded long after the Viking Age.
Its visual form, when described or depicted at all, varies considerably from source to source. Unlike most ancient Norse symbols, it doesn’t seem to have had any one definitive shape. There are also significant differences in how one would go about applying the Svefnthorn to someone, as well as the exact effects that the Svefnthorn would bring about once applied.
But the mentions of the Svefnthorn in the literature all have one thing in common: the Svefnthorn was used to put an adversary into a deep sleep from which he or she wouldn’t awaken for a long time.
Let’s review the mentions of the Svefnthorn in Old Norse and later Icelandic literature.
In The Saga of the Volsungs, the god Odin used a Svefnthorn to sink the valkyrie Brynhildr (Brunhild) into a slumber from which she would be unable to awaken until someone crossed the formidable circle of fire Odin had kindled around her sleeping body. The hero Sigurd was brave and capable enough to cross the flames to reach her, and so awoke her.
In The Saga of King Hrolf Kraki, Queen Olof “stuck” King Helgi with a Svefnthorn in order to knock him unconscious so that she could play a trick on him and his men. The effect wore off, apparently entirely on its own, in a matter of hours.
In Gongu-Hrolf’s Saga, Vilhjalmr stuck a Svefnthorn into Hrolf’s head in the night, and he didn’t wake up until well into the following day when a horse rolled his sleeping body around and the thorn fell out. Here, the thorn seems to have been a physical thing of some sort that was lodged in Hrolf’s head rather than “merely” a spell.
The ninth spell from the Huld Manuscript, an early modern book of Icelandic spells, references the Svefnthorn and reads, “This sign would be carved on oak and laid under the head of the one who is supposed to sleep so that he can not awaken until it is taken away.” The symbol that looks like a row of four harpoons (as in the picture at the beginning of this article) is drawn below.
Another relatively old spell from Iceland reads:
Sleep thorn: Take the heart sac [pericardium] of a dog; pour pickling broth into it. Then dry it for thirteen days long, in a place where the sun does not shine on it, and when the one to whom you wish to do this is asleep, hang this in the house over him completely without his knowing it.
The specific effects that the spell will bring about aren’t mentioned. How long will the sleeper remain asleep? Will he be able to awaken on his own or only at some appointed time?
In most of these cases, the visual representation of the Svefnthorn isn’t mentioned. When it is, it’s sometimes the “four harpoons” symbol from the above picture, and sometimes it’s a vertical line with a diamond shape at the bottom. This latter symbol could be an Isaz rune with an Ingwaz rune below it. Other Norse/Germanic symbols such as the Helm of Awe also seem to be comprised of runes. But the significance of this particular runic combination in connection with the Svefnthorn, if any, is unknown, and I’m not going to hazard a guess on this one.
So it’s not clear what the Svefnthorn’s visual form was or how one would go about applying it to a person. The sources contain contradictory information on this point, as they do on the point of whether the spell would wear off on its own or whether it could only be broken by some particular action being performed. In all likelihood, this is simply due to variations in all of these things across time and space. After all, ancient Germanic religion consisted of a set of common underlying ideas that were never codified or systematized, but varied greatly in their manifestations and applications. They weren’t as concerned with rationalization as we are today. But it is clear that the Svefnthorn was perceived to be a powerful magical tool that could put someone into a long and deep sleep from which he or she would have great difficulty awakening.
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 Völsunga saga. Chapter 20.
 Hrólfs saga kraka. Chapter 7.
 Göngu-Hrólfs saga. Chapters 24-25.
 Flowers, Stephen, editor and translator. 1989. The Galdrabók: An Icelandic Grimoire. p. 86.
 Ibid. p. 103.
 Ibid. p. 47.