What dragon gnaws the roots of Yggdrasil the world tree?

The dragon that gnaws the roots of Yggdrasil, the Norse world tree, is Níðhöggr.

Níðhöggr is described in Norse sources as a serpent-like dragon dwelling beneath Yggdrasil. In Snorri Sturluson’s Prose Edda, the root over Niflheim is gnawed from below by the creature, while the spring Hvergelmir lies beneath that root. The Poetic Edda’s poem Grímnismál likewise describes Níðhöggr biting the tree from underneath.

The monster is not merely a tree-eating dragon. Níðhöggr is also associated with the dead and with Náströnd, a grim hall described in Völuspá. There, it feeds on corpses, especially those of oath-breakers, murderers, and other offenders. Its name is commonly interpreted through Old Norse elements connected with biting, striking, or malicious disgrace, though the exact etymology remains debated.

Ratatoskr is the frequent mix-up: that squirrel runs through Yggdrasil carrying hostile messages between Níðhöggr below and the eagle above. Jörmungandr is Loki’s world-encircling serpent, while Fáfnir is a dragon from the Völsung cycle, not Yggdrasil’s root-gnawer.