The Nibelungs were the descendants of King Nibelung, a legendary Scandinavian king, and heirs to a vast treasure of gold and jewels that had been amassed in some ancient time.
Views tend to differ on the legend.
They gathered this great treasure that was eventually stolen from them and then proceeded to pass through a variety of owners. The hero in this saga, Sigurd, eventually acquires the gold by slaying Fafnir the dragon who guards the treasure. Fafnir had killed Sigurd's father Hreidmar to steal the treasure, and then turned himself into a dragon in order to protect the hoard from other thieves.