Although Nibelungs refers to the royal family of the Burgundians in the second half of the Nibelunglenlied (as well as in many other texts), in the first half Nibelungenlant it is instead a kingdom on the borders of Norway of which Siegfried becomes the ruler.
In Adventure 3 Hagen tells how Siegfried came by chance upon the two sons of the king of the Nibelungs who had just died.