I, like Jack, have always been interested in the Norse mythology and folklore. Elves, dwarves and dragons people are already familiar with, but the draugr (pl. draugar) are one of the least known aspects.
They’re basically the undead. “Draugr” often gets glossed as “ghost” but that’s not quite right because they have physical form and enough strength to snap a man in half. In a few of the stories they can do things like shape-shift and cast magic spells.
That’s crazy interesting!
Ulfdraugar, on the other hand, are completely Jack’s own invention. I think it’s appropriate that Jack would be misguided enough to pick such an obscure and hard to explain part of Norse folklore to leave his stamp on.