Nidhogg resides on one of the three branches of the tree Yggdrasil (Old Norse: Níðhöggr; which can be translated as "he who strikes full of hatred"), in Norse mythology according to the Völuspa, he is a dragon like creature who lives in the world of Niflheim where one of the roots of the Yggdrasil tree grows, he gnaws on the roots endlessly until Ragnarök comes and everything has vanished.
Nidhogg is always in conflict with the eagle that dwells atop Yggdrasil. The squirrel Ratatoskr, who carries messages between them and brings peace between the two.
Vikings who did not die in battle, according to Norse mythology, go to the realm of the dead, which was ruled by Hel, Loki's daughter. But before they get there, they have to pass by Nidhogg. Nidhogg sucks all the blood out of all the people who pass by, so that they turn completely pale.
Once all the blood has left them, it is easier for Hel to get them into her army of the dead, who will attack the humans when Ragnarok comes. Ragnarok will come on the day Hel has completed building her ship, Nagelfar. The ship is mostly made from the nails of the dead.