Níðhöggr (anglicized as Nidhogg) is a dragon in Nordic mythology that lives underground. In the old nordic, its name means "malice striker." It survives by eating the third root of the World Tree, Yggdrasill. This root resides over Niflheim, cold and dark world with perpetual mist. Nidhogg's relation with the root is complex, as sometimes it is said to trap the dragon in place. It's prison is also it's sustenance.
Nidhogg has notably bitter relation to an unnamed eagle that sits atop Yggdrasill. While the two cannot directly encounter each other, a squirrel called Ratatoskr is carrying messages from eagle to dragon, and vice versa. Both seem to be aggravated by this exchange of words.
One of the omens of impending Ragnarök is when Nidhogg is freed to roam the world.