The Niebelungenlied is a combination of two older epics. The earliest manuscripts of the (combined) epic date from around 1200-1205. The key motifs are vengeance, spectacular violence, doom, gloom and destruction. (There's precious little 'mystique').
Many German nationalists just loved the Nibelungenlied. In 1860 a fervent nationalist wrote:
Brach Etzels Saal in Glut zusammen,
Als er die Nibelungen zwang,
So soll Europa stehen in Flammen
Bei der Germanen Untergang!
(English: Just as Attlita's hall collapsed in glowing embers when he defeated the Nibelungs, so Europe will go up in flames if the Germans perish).
The Nazis turned this into a shorter slogan ... and went a long way to acting it out for real.