What dragon gnaws at the roots of Yggdrasil?

The story behind the answer

Nidhogg (Old Norse Níðhöggr) is the dragon-serpent that gnaws at the roots of Yggdrasil, the world tree connecting the Nine Realms. In the Eddas, he chews the root near the well Hvergelmir in Niflheim, threatening the tree’s strength.

By contrast, Fafnir is a hoard-guarding dragon slain by Sigurd, and Jörmungandr is the Midgard Serpent encircling the world; “Vorpalion” isn’t Norse. Nidhogg also feasts on the dead in Náströnd, while the squirrel Ratatoskr ferries insults between him and the eagle atop Yggdrasil—imagery of tension and decay within the cosmos. Memory tip: Nidhogg “hogs” (gnaws) the roots of the tree.