The Valkyrja's dental markings are based on bodies in Viking graves with horizontal lines carved into the teeth. The practice was particularly common on the island of Gotland. The principal theory is that the carvings were decorations, filled with colored pigment.
In the scene in which the Úlfhéðnar attack the Slavic town, Amleth catches a spear in midair and throws it back at the Slavs in one movement. This is taken from the medieval Icelandic story of Njáls saga in which Audolf throws a spear at the Viking hero Gunnar, but Gunnar catches it in midair and throws it straight through Audolf and his shield.
The plot is very loosely based on the story of Amleth, which appears in the Gesta Danorum (History of the Danes), a collection of oral traditions written around 1200 by Saxo Grammaticus. Amleth inspired Shakespeare's Hamlet, though it is doubtful that Shakespeare read Grammaticus directly.
Director Robert Eggers worked with historians and did meticulous research into the period, to make sets, costumes and props as authentic as possible, while also going back to a period before Christianity influenced Viking culture. Professor Neil Price, archaeologist at Uppsala University in Sweden who worked on the movie, later stated that "the Northman might be the most accurate Viking movie ever made." Eggers did admit that there were few historic references for Amleth's initiation ritual, making it "probably the most fictional ritual in the movie."
Although director Robert Eggers didn't have final cut and had to make several studio-mandated changes to the movie during post-production, he still expressed his satisfaction with the finished product. However, he admitted that editing the movie was the most painful experience of his life, making this possibly a one-off experience. "Frankly, I don't think I will do it again. Even if it means, like, not making a film this big ever again. And, by the way, I'd like to make a film this big. I'd like to make one even bigger. But, without control, I don't know. It's too hard on my person."
Robert Eggers: [folkloric, mythological, and fantastical period piece] La sorcière (2015), Le Phare (2019), this film, and Nosferatu (2024) are period pieces that are based on folklore, mythology, and fantasy, which reflect through their atmospheres, settings, and environments.