In Norse mythology, Níðhöggr is the serpent or dragon said to gnaw beneath the roots of Yggdrasil.
Níðhöggr dwells below the world tree and feeds on corpses, including the dead associated with Náströnd in descriptions of the underworld. The creature’s actions contribute to the imagery of decay beneath the cosmic order.
Níðhöggr is also part of a conflict involving the eagle at the top of Yggdrasil and the squirrel Ratatoskr, who carries hostile messages between them. This creates a vertical chain of tension running through the world tree.
Níðhöggr is often confused with Jörmungandr, the Midgard Serpent. Jörmungandr encircles the world and is Thor’s great serpent enemy, whereas Níðhöggr is linked to Yggdrasil and the dead.