The Collaborative International Dictionary of English v.0.48:

Nibelungs \Ni"be*lungs\, n. pl.; sing. Nibelung.
   In German mythology, the children of the mist, a race of
   dwarfs or demonic beings, the original possessors of the
   famous hoard and ring won by Siegfrid; also, the Burgundian
   kings in the Nibelungenlied.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Nibelung
    n 1: (German mythology) a companion or follower of Siegfried
    2: (German mythology) any of the race of dwarfs who possessed a
       treasure hoard that was stolen by Siegfried