Nibelungenlied is an epic in Middle High German. It is the work of an anonymous 13th century Austrian poet from the Danube area. He re-worked various pagan heroic motifs into a work of courtly poetry.
The word Nibelungen has several meanings.
Sigurd proposes to Kriemhild, the beautiful sister of Gunther, Gernot and Giselher, three Burgundian kings. He is allowed to marry her after he defeats Brünhild, the queen of Iceland, with the aid of a Tarnhelm and she becomes Gunther's wife.
Kriemhild lets slip the secret in a row with Brünhild, and Hagen decides to kill Sigurd. He finds out his most vulnerable spot and kills him while out hunting.
Attila the Hun proposes to Kriemhild, and she invites the Burgundians to a feast in Hungary. There is a huge fight, and everyone is killed except Gunther and Hagen who are captured by Dietrich of Bern.
Kriemhild demands the return of the Nibelungen treasure (stolen by Hagen). When she fails to get it back, she arranges for Gunther to be killed and cuts off Hagen's head with Sigurd's sword. She in turn is killed by Hildebrand, Dietrich's armourer.