Nidhogg (Norse Níðhöggr) is a ferocious dragon who gnaws at the roots of Yggdrasil, the tree which supports the nine worlds of Norse mythology. This power-hungry monster is sometimes referred to as “the Malice Striker,” an appropriate name given that he rules over dark criminals and is bent on destroying peace and virtue.
Nidhogg is a tremendous dragon. His body is covered in bright scales, and horns erupt from his head. A pair of forelegs, complete with massive claws, help him to rip at the roots of Yggdrasil, but he has no back legs, only a serpentine tail. Beneath his bat-like wings, he carries the corpses of criminals.
His mammoth body can be found twisting through the roots of Yggdrasil, especially around Niflheimr, the cold world from which all the rivers of Midgard spring. Occasionally, he might slither into Hel to visit the dark goddess who some people consider his master.
Balance is extremely important in Norse mythology, and while Nidhogg does represent a ghastly force, he is still important to supporting the balance of Yggdrasil. A great eagle, who represents wisdom and virtue, perches in the uppermost branches of the tree, while Nidhogg, representing chaos and evil, lurks in its roots. The constant tension between the eagle and the dragon is fueled by Ratatoskr, a squirrel who runs up and down the tree ferrying insults between the two enemies. This tension may seem undesirable, but it actually promotes a cycle of growth in the tree of life. After the eagle and the dragon spend the day destroying Yggdrasil in their frenzy to attack each other, the tree is bathed in water from the wells of Urd, which promotes healing and new growth.
In addition to bringing balance to Yggdrasil, the monster also figures in the punishment of criminals. He rules over the dark shores of Nadastrond, to which the corpses of murderers, adulterers, and oath-breakers are banished. A terrifying hall, with walls woven from serpents and a ceiling that drips venom, waits for these criminals, and inside the hall, the dragon chews on their bodies.
Finally, the dreaded dragon has a role to play in Ragnarok, the day when the giants will attack the gods and destroy most of their world. Ragnarok will begin when the dragon finally manages to chew through the roots of Yggdrasil, causing the tree to yellow and the worlds it supports to plunge into a three-year winter. At the end of this frigid and chaotic period, he will fly up from the underworld, carrying dead criminals and leading the giants on an attack against the gods. Ultimately, the he will survive this battle and become the force of evil which balances good in the post-Ragnarok world.
Sources: mythology.net, Sons of Vikings