The dragon that lives in the roots of Yggdrasil is Níðhöggr, a serpent-like creature that gnaws at the world tree.
Níðhöggr is described in the Poetic Edda and in Snorri Sturluson’s Prose Edda. One of Yggdrasil’s three great roots reaches toward Niflheim, and Níðhöggr gnaws at it from below. The dragon’s destructive activity contributes to the recurring image of Yggdrasil as a living cosmic structure exposed to continual decay and attack.
Níðhöggr is also associated with the dead and with the realm of Hel. In the poem Völuspá, it appears in imagery connected with Náströnd, a grim place inhabited by wrongdoers. A squirrel named Ratatoskr runs up and down Yggdrasil carrying hostile messages between Níðhöggr below and an eagle near the tree’s summit.
Names such as Surt, Jörmungandr, and Fáfnir belong to other famous beings. Jörmungandr is the Midgard Serpent, Fáfnir is a cursed dragon, and Surt is a fire giant.