Nibelung myth is part of the general stock of German mythology. It was spread by oral tradition in many differing versions, some of which were written down in monasteries and passed on.
Its main figures are Siegfried, Kriemhild, Gunther, Brunhild and Hagen von Tronje. It's a fascinating mix of ancient mythology (dragons, dwarves, Nibelung treasure and magic devices) and Christian beliefs (the supremacy of God, compassion and marriage).
Many poets and musicians have been fascinated by the Nibelung theme and have adapted it to their time and age. A famous example for this is Richard Wagner's epochal opera cycle "Ring of the Nibelung".