The Nibelungenlied is based on pre-Christian Germanic heroic motifs (the "Nibelungensaga"), which include oral traditions and reports based on historic events and individuals of the 5th and 6th centuries.
In this poem, everything turns around the Ring of The Nibelungs. This Ring, among all the other treasures the Nibelungs possess, is the most important to them, the most loved. They warn Siegfried, that, as he is the slayer of the Dragon, he can legally claim all their treasure apart from the ring. But he thinks that the ring may have some supernatural powers, and as being blind by his greed, he takes it as well. He did not listen to the Nibelungs' warning and the ring doomed him. His fate was from now on doomed to a bad and quick end.
The similarity with Tolkien's masterpiece: the Lord of The Rings is, apparently, very obvious. In the First Days, in The Great Battle where Elves and Men were allies (the Days of The Last Alliance), when Isildur, son of Elendil, took revenge for his father's death and cut the finger of Sauron which had the Ring of Power, he managed to almost destroy the evil spirit of Sauron. He would have totally vanished evil from Middle-Earth, but he was greed, and the power of the ring made him blind. So, when he and Elrond, the Half-Elven, reached the depths of Mount Orodruin, regardless all the urges from Elrond, Isildur did not throw the ring into the roaring fires of the mountain, the only place where it could be destroyed. Thus, Isildur found a tragic end because he kept the Ring. His fate was also doomed by a ring and by the desire to possess something he shouldn't have.
The Nibelungenlied is the main axon upon which Tolkien based The Lord of the Rings. The Ring of the Nibelungs gave him the inspiration for the Rings of Power made by Sauron and the Elves and eventually lead to the creation of one of the best works of authorship of his era!