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.]