Níðhöggr in Norse mythology is a creature that gnaws at the roots of the World Tree Yggdrasil. He lives beneath the roots of Yggdrasil and gnaws at them as they are the only thing that keeps it from the world. The squirrel Ratatoskr brings words of envy between Níðhöggr and the eagle Hresvelgr. The only person capable of controlling him is the Goddess Hel. Its existence is the portend of the Ragnarok that signals the end of all.
Nidhoggr appears as a optional boss in the Tower of Daedalus. Defeating him rewards the Devil Analyzer, which shows info about the last five demons that have been encountered.
Digital Devil Saga: Avatar Tuner
Nidhogg appears as a late game enemy in Digital Devil Saga: Avatar Tuner.
Persona 3 FES
"An evil dragon that gnaws on the roots of the Yggdrasil, the World Tree. It rules over the evil snakes that live there. Capable of surviving Ragnarok by feeding on the slain corpses that drift to it."