Nidhogg is the name of the dragon/serpent known for inhabiting the world of Niflheim, coiling around the well of Hvergelmir (from which all waters are supposed to flow), and chewing on the roots of the World Tree (Yggdrasil). The only times when it is not viciously attacking the roots, and forcing the tree to decay and regrow, is when it is feeding upon the corpses of the dead at the shores of Nastrond(Hence why Nidhogg often gets the names: "lower one", "full of hatred", "dread biter") and when wyrd is spoken by a volva. Oh, and when he pauses to send hateful messages up the tree via Ratatosk (the squirrel) to the Eagle sitting at the top, or when in turn Ratatosk brings him a hateful message back from said Eagle.
" Nidhogg is the chthonic counterpart of the aquatic serpent Iormungand, but unlike her, and all preceding Rkkr beings, the origins of Nidhogg are totally obscure. The dragon, like the World Tree on which it chews, appears to have arisen out of Wyrd itself, having no creation, no creator. Suggesting at least that they both began their existence so remotely in the subconscious that they have gone beyond the need for any explanation. Certainly, Nidhogg has gone beyond the need for definition. It is beyond gender, it is neither feminine, nor masculine, but not like the undivided lunar nature of Hela, or the androgynous solar nature of Loki. Instead, its nature is so incomprehensible, so chaotic, that it is simply beyond any such definition.Consequently the element associated with Nidhogg is Chaos ."
(http://shadowlight.gydja.com/nidhogg.html)
Nidhogg is not the only serpent down there either, though certainly the most well known and monstrous. I think somewhere back I wrote a thread on all the inhabitants involved with Yggdrasil, and listed the names of a few other serpents. Along with Nidhogg, four stags (named Dainn, Duneyr, Dvalinn and Durathor) gnaw the leaves/bark of the World Tree. This eventually shakes it to its core during Ragnarok, which is when Nidhogg supposedly comes up from Niflheim, bringing the dead into battle. However, it suggests that Nidhogg continues on after Ragnarok, and is not one of the beings who is killed in battle.
Nidhogg really has no personal descriptions, no descriptions of how it appears, as a lot of it is vague and unclear- left up for personal interpretation. Symbolism involving Nidhogg interests me. I had heard someone theorize that Nidhogg was meant to represent evil, and that by Nidhogg living on after Ragnarok it meant evil was still, and would always be, eternal and involved in the world. However, I really think that's looking at it from a black/white perspective. I don't personally think Nidhogg is either good nor bad, just some strange combination beyond our grasp of understanding.
NorthernPaganism.org, which I do believe is run by Raven K and some other people, suggests that Nidhogg plays an important role in Rokkatru and teaches an important lesson: "Nidhogg's Rule : Recycling. There is no such place as Away, so be careful what you throw there. Recycle, give away, don't waste, find a place for it somewhere else. This includes people as well as things. "
You can incorporate Nidhogg into a magickal practice, if you wish, though I do not know many Heathens who have actually done much in-depth work with Nidhogg. Nidhogg is the kind of being that doesn't interact much with those who work with it. You can leave offerings for Nidhogg. The most appropriate I have seen would be decomp bins, where you leave food to rot and become part of the soil. Since Nidhogg represents decay, this is an important aspect. Dead animal parts would be an appropriate offering as well, as Nidhogg eats the dead. I have heard of some Heathens incorporating Nidhogg into Chaos Magick, but I have no personal experience with that.