Possible Clues:

• Old Norse poems from some unremembered days
• Ancient work that describes the sacred tree Yggdrasil
• One literary source for Wagner's "Ring" cycle
• Classical Icelandic literary work
• Inspired, Dante pens literary collection
• Medieval Icelandic work
• Icelandic anthology
• Somewhat revered, dark collection of legends
• A primary source for Scandinavian mythology
• Inspiration for Wagner's Ring Cycle