The most important thing about Níðhöggr is that he never wins and never stops.
He lies beneath the third root of Yggdrasil, in Hvergelmir, chewing. Grímnismál says more serpents lie under the ash than any fool imagines — and names some of them: Góinn and Móinn, sons of Grafvitnir, Grábakr, Grafvölluðr, Ófnir and Sváfnir — and that they will forever gnaw the tree's limbs. He is the largest of them.
That is a cosmology with structural decay built into it as a permanent operating condition, not as an approaching disaster. The tree is chewed from below every day of its existence, and it stands because the norns water it every day of its existence. Neither side is winning. This is what "the world" means in Norse thought.
Nid, and why the name is an accusation
Níðhöggr is usually rendered "malice-striker" or "the one who strikes with spite." The first element, níð, is not a vague word for evil: in Old Norse it is a precise legal and social term for a specific kind of dishonour, the accusation of cowardice or unmanliness that could be prosecuted, and níðstöng was a carved insult-pole raised to curse an enemy. A man declared níðingr had lost his standing entirely.
So the dragon at the root is named after the worst social offence the culture recognised, and his diet reflects it.
What he eats
Völuspá stanza 39 describes a hall on the corpse-shore, Náströnd, with doors facing north, woven of serpents' spines, with venom dripping in through the smoke-hole. Wading the heavy streams there are perjurers, murderers, and men who seduce another's wife — and Níðhöggr sucks the corpses of the dead while the wolf tears men apart.
This is the closest thing in Norse literature to a hell with a moral entry requirement, and it lists three offences, all of which are legal categories in Icelandic law rather than sins in a theological sense. Oath-breaking heads the list.
The authenticity of that stanza has been debated for a long time. It sits in a poem that elsewhere shows possible Christian colouring, it describes a punitive afterlife that the rest of the corpus does not support, and its position varies in the manuscripts. Whether a pagan Norse audience believed oath-breakers were eaten after death, or whether a Christian-era poet supplied a hell because his material lacked one, is not settled — see Hel for how unlike this the ordinary Norse afterlife is.
The last stanza of Völuspá
The poem ends, after the new world has risen and the surviving gods have gathered, with an image nobody has fully explained.
A dark dragon comes flying from below, from Niðafjöll, the dark mountains. Níðhöggr bears corpses in his wings as he flies over the plain. And then the seeress sinks.
That is the final stanza of the greatest poem in Old Norse, and it comes after the renewal — after the fields grow unsown, after Baldr returns, after the description of a hall roofed with gold where the righteous will live forever.
The interpretations divide sharply. One reads it as a survival of the old order into the new, a note of unresolved darkness deliberately placed last. Another reads it as a displaced stanza belonging earlier in the poem. A third reads the dragon's departure as the removal of the dead — carrying them away, ending the corpse-eating. The Codex Regius and the Hauksbók versions of Völuspá differ in ways that keep the argument open.
Nobody has resolved it, and its ambiguity is a large part of why the poem is still read.
Not a hoard-dragon
Níðhöggr guards nothing, hunts nothing and speaks to nobody except through Ratatoskr, the squirrel who carries insults between him and the eagle in the branches. He is not a Fáfnir, who is a transformed man lying on treasure — see Sigurd — and not a Jörmungandr, who at least has an appointment at the end of the world.
Norse dragons generally are worth distinguishing from the later European kind: they are ormar, serpents and worms, without wings in most descriptions. Níðhöggr flies in the last stanza, which makes him unusual in the corpus and is another reason that stanza gets argued about. See dragons and serpents across the traditions.
The root-gnawer as album art
The name has been taken up enthusiastically by metal bands, games and fantasy fiction — usually as a straightforward evil dragon, which loses the point. In God of War he appears as a boss. The Icelandic form Níðhöggur remains in occasional use for boats and businesses.
For the tree he is destroying, see Yggdrasil; for the well he lies in, Niflheim; and for the tradition, the Norse overview.
Is Níðhöggr trying to destroy the world?
Nothing says so, and the framing is probably wrong. He is not described as an enemy of the gods, he takes no part in the battle at Ragnarök, and no one opposes him — no god is assigned to fight him, which is remarkable in a mythology that pairs off almost every major antagonist. He gnaws because that is what he does. The Norse cosmos does not require its destructive forces to have intentions, any more than rot does.
Where exactly is he: under the tree, or at the corpse-shore?
Both, in different texts, and there is no reason to think a medieval audience needed them reconciled. Grímnismál has him at Hvergelmir under the third root; Völuspá has him on Náströnd with the dead; the closing stanza has him flying up from Niðafjöll. Snorri places Hvergelmir in Niflheimr and the dead's realm below, which brings the locations close together without joining them. Norse cosmic geography is consistently a set of directions — down, north, out — rather than a map, and demanding coordinates from it produces false precision.