Nídhögg

In Norse mythology, Níðhǫggr (Malice Striker, often anglicized Nidhogg) is a dragon 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 honor 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 standardized Old Norse orthography, the name is spelled Níðhǫggr, but the letter 'ǫ' is frequently replaced with the Modern Icelandic ö for reasons of familiarity or technical expediency.

The name can be represented in English texts with i for í; th, d or (rarely) dh for ð; o for ǫ and optionally without r as in Modern Scandinavian reflexes. The Modern Icelandic form Níðhöggur is also sometimes seen, with special characters or similarly anglicized. The Danish forms Nidhug and Nidhøg can also be encountered; or Norwegian Nidhogg and Swedish Nidhögg.

According to the Gylfaginning part of Snorri Sturluson's Prose Edda, Níðhǫggr is a being which gnaws one of the three roots of Yggdrasill. It is sometimes believed that the roots are trapping the beast from the world. This root is placed over Niflheimr and Níðhǫggr gnaws it from beneath. The same source also says that "[t]he squirrel called Ratatoskr runs up and down the length of the Ash, bearing envious words between the eagle and Nídhǫggr [the snake]."

...
Wikipedia