The Nibelungenlied or The Song of the Nibelungs, is an epic medieval  German poem tells the story of dragon-slayer Siegfried at the court of the Burgundians, his murder, and of his wife  revenge.
Like most European epic poetry of the era the written work is based on earlier oral stories.  In the case of the Nibelungenleid, the stories are pre-Christian Germanic heroic myths that date from the 5th and 6th centuries. Similar versions of the legend survive in the Völsunga saga, the Prose Edda, and the Poetic Edda.
There 35 known manuscripts of the Nibelungenlied. Eleven of these manuscripts are essentially complete, and 24 are in various fragmentary states of completion and in a variety of versions of the story. A note on the title of the poem, ‘Lied’ is song in modern German, but in the middle high German that the poem was written in it probably was a translation of the Norse word ‘lay,’ which means tale or epic.
The written Nibelungenlied is generally agreed to be the work of an anonymous poet who lived between Passau and Vienna, dating from about 1180 to 1210, possibly at the court of the bishop of Passau, Wolfger von Erla (in office 1191–1204).

The Nibelungenlied

The Nibelungenlied is a tragic work that can be divided into two parts, the first dealing with the story of Siegfried, his past as a valiant knight which involved winning a treasure and lands from a pair of brothers, Nibelung and Schilbung, whom Siegfried had killed when he was unable to divide the treasure between them and the heroic killing of a dragon. After killing the dragon, Seigfried had bathed in its blood rendering him invulnerable. Unfortunately a leaf fell onto his back from a linden tree as he was bathing in the blood and the tiny patch of skin that it covered did not come into contact with the dragon's blood, so that Siegfried remains vulnerable in that one spot (similar to the  Greek hero Achilles and his vulnerable heel.) The first part continues with Siegfrieds winning of the hand of Kriemhild, the wooing of Brünhild by King Gunther, the death of Siegfried at the hands of Hagen, and Hagen's hiding of the Nibelung treasure in the Rhine. The second part deals with Kriemhild's marriage to Etzel, her plans for revenge, the journey of the Nibelungs to the court of Etzel, and their last stand in Etzel's hall.
Real events are conflated with common Germanic myths. The Nibelungenlied combines a mythological stories such as Gunther's wooing of Brünhild, but place the stories in real locations such as Worms, the capital of Burgundy, real events such as the journey of the Nibelungs east across the Danube to Etzelburg.
The Nibelungenlied mixes these Germanic myths with more modern ideas of courtly Christian chivalry. Consequently, Siegfried changes from a dragon killer to a courtly man who will express his love to Kriemhild explicitly only after he has won the friendship of the Burgundian king Gunther and his brothers, Gernot and Giselher. However the bloody end of the epic is far removed from the happy ending of typical French courtly epics.
The Nibelungenlied, Thidreks saga and the Völsunga saga served as source materials for Richard Wagner's Der Ring des Nibelungen (English: The Ring of the Nibelungs), a series of four music dramas popularly known as the "Ring Cycle".