It can only be drawn from its sheath by night, or at the “Gates of Hel” [SPOILER WARNING: that ends up being inside a volcano], and it craves blood. [And boy does it get it!] The sword is called Draugr, which is also what one calls the mound-dwelling revenants of Nordic mythology; Amleth, hero of THE NORTHMAN, has to fight the zombie to gain possession of Draugr the sword. [SPOILER WARNING: he does.] Once he has the sword, he uses it to wreak some seriously bloody havoc. Like, seriously.
The amount of research Robert Eggers did for THE NORTHMAN is staggering. Almost everything in it, down to the most minute detail, has an historical or mythic antecedent or inspiration. Draugr the Vampire Sword (for such I will henceforth call it) is probably based on Skofnung, supernatural sword of King Hrolf Kraki of Denmark. And upon the sword of Grettir the Strong, both of which were recovered from burial mounds.
Think of Draugr as Excalibur with a bad attitude. And when they said it couldn’t be drawn in the sun, they weren’t kidding. Draugr flat refuses to come out of the sheath during the daytime. I hope they make a (relatively inexpensive) Draugr replica for collectors, because I wants one!