We don't have much evidence for beheading in Viking Age Scandinavia. There's an exceptional cemetery in the far north of Norway, where it seems like slaves brought in from elsewhere were sometimes beheaded as human sacrifices in fancy burials. There's also the story of Regin who gets beheaded in the Saga of the Volsungs. A Viking-Age image of this is preserved on the Ramsund runestone that was raised in Sweden around 1030. But these are simply examples of beheading, and the placement of the head doesn't seem to have been important in either case.

Conversely, vikings abroad encountered some pretty nasty practices of beheading in Western Europe. Ninth-century Irish chronicles record defeated vikings getting beheaded in 852, 863, 865, and 866. A famous excavation at Ridgeway Hill in Dorset in southern England turned up what looks like a viking crew that was captured and executed. Counting the bones was a bit of a mess, but it seems like there were 52 bodies in one pile and 48 skulls in another, suggesting that the Anglo-Saxon victors took some gruesome trophies home. Of course, vikings were also adaptive to their circumstances, and the Hiberno-Norse kings of Dublin mounted skulls on their town wall. Again, the placing of the heads doesn't seem to have mattered so much as the actual act of beheading, but the evidence is scant.

So what might have inspired the movie writers to include this particular piece? I suspect they were inspired by the work of Andrew Reynolds on Anglo-Saxon deviant burials. These seem to have been the judicial executions of criminals taking place late in the Viking Age (so perhaps an inspiration for the Ramsund stone as well?). Perhaps the most famous set of these burials was excavated at Sutton Hoo, where East Anglian kings were buried until about 700, but then a few dozen individuals were buried in strange positions perhaps a few hundred years later. There's some images of the excavations available here. One of the Sutton Hoo burials had the head between the individual's legs (not enough skeletal material was preserved to discuss biological sex). In Reynolds' larger catalog of deviant burials, decapitations are pretty common, with heads often between the legs (only in two cases between the thighs) or sometimes beside the thighs, perhaps with the individual buried on their side. It is possible that these placements represent the haphazard tossing of a criminal's remains into a grave, but some archaeologists have speculated that the bodies were position to communicate an enduring sense of dishonor as well. So maybe Anglo-Saxon kings were doing this thing. Maybe.

Does that mean The Northman is building on solid history? Absolutely not. But it is a fun movie that draws inspiration from early medieval archaeology and history, and that at least leads to interesting questions!

In Grettir's Saga, the titular character defeats a draugr after an exhausting night-time battle, and cuts its head off. He then put it at the draugr's backside as a temporary measure to keep it from rising again. Once he's rested a bit, he and the farmer who was hosting him for the night then burn the body to fully get rid of the creature.

A maybe interesting detail to add - the whole scene is based on something described in the Icelandic Saga of Grettir the strong, chapter 18:

Then Grettir entered into the barrow, and right dark it was, and a smell there was therein none of the sweetest. Now he groped about to see how things were below; first he found horse-bones, and then he stumbled against the arm of a high-chair, and in that chair found a man sitting; great treasures of gold and silver were heaped together there, and a small chest was set under the feet of him full of silver; all these riches Grettir carried together to the rope; but as he went out through the barrow he was griped at right strongly; thereon he let go the treasure and rushed against the barrow-dweller, and now they set on one another unsparingly enough. Everything in their way was kicked out of place, the barrow-wight setting on with hideous eagerness; Grettir gave back before him for a long time, till at last it came to this, that he saw it would not do to hoard his strength any more; now neither spared the other, and they were brought to where the horse-bones were, and thereabout they wrestled long. And now one, now the other, fell on his knee; but the end of the strife was, that the barrow-dweller fell over on his back with huge din. Then ran Audun from the holding of the rope, and deemed Grettir dead. But Grettir drew the sword, 'Jokul's gift,' and drave it at the neck of the barrow-bider so that it took off his head, and Grettir laid it at the thigh of him. Then he went to the rope with the treasure, and lo, Audun was clean gone, so he had to get up the rope by his hands; he had tied a line to the treasure, and therewith he now haled it up.

Like all Sagas it was written down at least 200 years after its events supposedly took place and it was never intended to be a factual historical account. But still an interesting and odd mention of this practice.