Prose EddaAccording to the Gylfaginning part of Snorri Sturluson's Prose Edda, Níðhöggr or "Nidhogg Nagar" is a being which gnaws one of the three roots of Yggdrasill. It is sometimes believed that the roots are trapping the beast from the world. This root is placed over Niflheimr and Níðhöggr gnaws it from beneath. The same source also says that "[t]he squirrel called Ratatöskr runs up and down the length of the Ash, bearing envious words between the eagle and Nídhöggr."
In the Skáldskaparmál section of the Prose Edda Snorri specifies Níðhöggr as a serpent in a list of names of such creatures:
Snorri's knowledge of Níðhöggr seems to come from two of the Eddic poems: Grímnismál and Völuspá.
Later in Skáldskaparmál, Snorri includes Níðhöggr in a list of various terms and names for swords.
Poetic EddaThe poem Grímnismál identifies a number of beings which live in Yggdrasill. The tree suffers great hardship from all the creatures which live on it. The poem identifies Níðhöggr as tearing at the tree from beneath and also mentions Ratatoskr as carrying messages between Níðhöggr and the eagle who lives at the top of the tree. Snorri Sturluson often quotes Grímnismál and clearly used it as his source for this information.
The poem Völuspá mentions Níðhöggr twice. The first instance is in its description of Náströnd.
Níðhöggr is also mentioned at the end of Völuspá, where he is identified as a dragon and a serpent.
The context and meaning of this stanza is disputed. The most prevalent opinion is that the arrival of Níðhöggr heralds Ragnarök and thus that the poem ends on a tone of ominous warning.
Níðhöggr is not mentioned elsewhere in any ancient source.
Níðhöggr's nameIn the standardized Old Norse orthography the name is spelled Níðhǫggr or Niðhǫggr but the letter 'ǫ' is frequently replaced with the Modern Icelandic 'ö' for reasons of familiarity or technical expediency.
The name can be represented in English texts as Nidhogg, Nidhoggr, Nithhogg, Nidhögg, Nidhöggr, Nithhöggr, Nídhöggr, Nithhoggr, Nidhhogg, Níðhögg, Niðhoggr, Níðhoggr, Nídhögg, Hidhaegg, or Nidhhoggr. The Modern Icelandic forms Níðhöggur and Niðhöggur are also sometimes seen and anglicized as Nidhoggur. The Danish form Nidhug or "Nidhøg" can also be encountered.