According to the song Grímnismál, Nidhögg belongs to the creatures at the world tree Yggdrasil. On the one hand, he damages the tree down there by the trunk or the roots, and on the other hand, he receives the words that the squirrel Ratatöskr delivers to him from the eagle that sits in the crown at the other end of the tree.
The Prose Edda builds on this description, adding that Nidhögg lives, along with many serpents, in the spring Hvergelmir, located in Niflheim under the third root of the World Tree.
From there he gnaws at the root of Yggdrasil. In contrast to the Grímnismál, the Prose Edda also has the squirrel deliver messages from Nidhögg to the eagle and evaluates their dialogue as an exchange of spite.
In the creation song Völuspá, on the other hand, Nidhögg is not mentioned at all at the World Tree, but only in the Last Days (Ragnarök), when the murderers and the oath- and adultery-breakers at the dead beach Nastrand arrive at a hall entwined with serpents' backs, where Nidhögg drinks their blood. Moreover, insofar as the kenning "nose bleacher" stands for Nidhögg, he tears up the dead at that time.
After the Ragnarök is over, Nidhögg embeds the corpses in his wings and ascends with them from the underworld mountain Nidafjöll, then descends with them. Where he flies with them remains open. The nature of Nidhögg is described only here, namely as a snake and at the same time as a dragon.
In the Prose Edda a different version of this passage of the Völuspá is quoted. With the consequence that Nidhögg does not suck or tear anyone in the Ragnarök, but only stays after the Ragnarök in the spring Hvergelmir and tortures the dead there. The Prose Edda reports nothing about Nidhögg's flight or his nature.
Outside the Edda, Nidhögg is still mentioned in the Þulur. His name could therefore be used as Heiti (comparable to a poetic synonym) for snake.
In the Nordic cosmogony of the Middle Ages, Nidhögg is a dragon that feeds on the blood of the dead in or after the end times. At least this is how one must see him, if one understands the statements of the different texts as puzzle pieces of the same picture.
If one looks more exactly, however, at least two myth layers can be distinguished in the tradition of Nidhögg, which are not congruent and probably originate from different times.
In the Völuspá Nidhögg is described in his function as a dragon of the dead as if he had sprung directly from the Christian imagination of the Middle Ages.
The way from the serpentine dragon, which torments the dead in the end times and appears as the underworld adversary of the Allfather, is not far to the adversary of God, the great dragon, the old serpent called devil and Satan, who is vanquished by archangel Michael in the apocalypse of John.
Dragons of death are virtually typical in the apocalyptic vision poetry of the Middle Ages. The Prose Edda completes this equation by placing the dragon in the spring Hvergelmir, thus making it part of a Christian punishment fort.
It furthermore links this place with the New World after the End Times, while the Völuspá strictly leaves this open and one can only interpret it into it.
The nature of Nidhögg is not one hundred percent clear. In the Völuspá Nidhögg is called both nadr "serpent" and dreki "dragon." However, the flight of Nidhögg described in it points to a dragon, since snakes cannot fly.
Rudolf Simek uses this example to illustrate that apparently no clear distinction was made between snake and dragon until the high Middle Ages. Snake could always mean dragon as well.
Just from this it becomes clear that Nidhögg's nature has a great closeness to the snake, which is confirmed by the company of many snakes at his side.
The snake played a greater role in the Germanic world in pre-medieval times, especially in the Bronze Age, than at the time of the writing of the Eddas.
As a chthonic animal, it was close to ancient fertility cults, ultimately concerned with the return of life (spring) after death (winter). Possibly the serpent had a special position in the cult of the dead, as indicated by the dragon Nidhögg on the World Tree, but also by the world-embracing Midgard Serpent.
The Christian imprint of Nidhögg in the Völuspá can be used to interpret the description of his role on the World Tree in the song Grímnismál, but this text can just as easily stand on its own and describe another myth far older than the medieval coloring of Nidhögg.
The serpent or dragon at the roots of the tree of the world or tree of life are part of the basic framework of the mythology of many peoples. They appear not only in the Bible in the Garden of Eden, but also in Siberia and especially among the Indo-European peoples, for example in the figure of Ladon on the Greek tree of the Hesperides or in the Indian Nagas, who are in dispute with the eagle Garuda, who dwells above them in a tree.
There is another parallel to the quarrel on the tree in the Indo-European world: In a fable of the Roman poet Phaedrus, a cat on a tree causes enmity between an eagle in the height and a wild sow at the roots.
What the dispute between above and below is about, however, can no longer be precisely determined.
To understand the two Widerparts as opposite poles of the duality is possible, but perhaps only a modern interpretation, as the examples of Ladon against Heracles and of the Nagas against Garuda show, in which it is ultimately about the access to the means of immortality.
It is not generally accepted in scholarship, however, that Nidhögg represents an Indo-European heritage.