Níðhöggr, a dragon/serpent who gnaws at a root of the world tree, Yggdrasil. In historical Viking society, níð was a term for a social stigma implying the loss of honour and the status of a villain. Thus, its name might refer to its role as a horrific monster or in its action of chewing the corpses of the inhabitants of Náströnd: those guilty of murder, adultery, and oath-breaking, which Norse society considered among the worst possible crimes.
In the Skáldskaparmál section of the Prose Edda Snorri specifies Níðhǫggr as a serpent in a list of names of such creatures:
These are names for serpents: dragon, Fafnir, Jormungand, adder, Nidhogg, snake, viper, Goin, Moin, Grafvitnir, Grabak, Ofnir, Svafnir, masked one.
But better known to most of us Ace Combat Fans, as Grabacr and Ofnir.