Nidhogg is a creature from Norse mythology who chews on the tree Yggdrasill.

In Norse mythology, there is an immense ash tree called Yggdrasill. Its branches extend up into the heavens and it is supported beneath the earth by three roots. The first root goes to the home of the gods, Asgard, where they go every day to hold court. Urd�s Well, where the Norns live, is under this root. The second root goes to Jotunheim, the land of frost giants and trolls. Mimir�s Well, that contains all the wisdom in the world, is under this root guarded by Mimir. The third root leads Niflheim, where Hel rules the Underworld. Hvergelmir, the wellspring of cold and the source of all cold rivers, is located here. Above the spring lies the dragon Nidhogg, the dread biter.

Nidhogg is a monstrous serpent that relentlessly gnaws on the deepest root of Yggdrasill. He hopes to chew through the root causing the World Tree to fall and bring about the end of the world. He is not the only creature that eats away at Yggdrasill. There are four deer, Dainn, Dvalinn, Duneyrr, and Durathror, who eat the leaves and gnaw on the branches of the tree. Every day the Norns draw water from Urd�s Well and take sand that lies around it, which they pour over the World Tree to heal it, undoing all of the damage done to it by its inhabitants including Nidhogg.

Nidhogg lies upon Nastrond, the corpse shore. Nastrond is one section of Niflheim where those that did not die heroic deaths go. On Nastrond is a great hall of evil whose doors face north. The hall is made of serpents whose heads blow venom, forming rivers that run the length of the hall. Murderers, adulterers, and oath breakers are forced to wade through those rivers.

A loathing is felt between Nidhogg and a nameless eagle that lived at the very top of Yggdrasill. The eagle sits atop the World Tree and has knowledge in many things. Between his eyes sits a hawk. The squirrel Ratatosk runs up the tree to tell the eagle of every curse or insult Nidhogg utters about the eagle. If the eagle spoke any negativity toward the great serpent, which happened just as often, Ratatosk would run down Yggdrasill to tell Nidhogg of it. Everything that happens in Yggdrasill is reflected in the human world. Nidhogg is the cause of the world�s troubles and misery, the eagle is the world�s high ideals and joy and Ratatosk is the tension between the two.

When Mimir stops guarding his well, Yggdrasill's roots will begin to rot. This will allow Nidhogg to chew through the root that runs through Niflheim. Urd�s Well will become polluted and the World Tree�s leaves will yellow. A three-year winter will set upon the world before Ragnarok; the final battle between the gods, the monsters, and the humans begins. A branch from Yggdrasill will fall on the Midgard Serpent, causing him to let go of him tail. The wolves, Skoll and Hati will succeed in catching the sun and the moon. Fenrir the wolf and Garmr, who guards Hel, will break their chains. The giants will release Loki from his mountain prison.

Nidhogg will fly up from below Yggdrasill and head to Asgard. He will carry the dead on his wings to join in the battle and behind him will come the giants. Loki will lead the monsters and giants in an attack on the gods. In the end, the only ones left are two humans and a few gods including Odin�s brother Hoenir, Odin�s sons Vidar and Vali, Thor�s sons Modi and Magni. In one version of the story of Ragnarok Nidhogg is killed by Thor�s son Magni. In another version, Nidhogg survives the final battle to provide an evil balance to the new good.