Nidhogg (Nidhoggr, roughly "malice-striker") is named in both the Poetic Edda and the Prose Edda as a serpent or dragon at the bottom of the world-tree Yggdrasil, in or near the spring Hvergelmir, gnawing at one of its roots. The tree is thus under continual assault from below even as it upholds the cosmos, an image of decay working against order. In the Grimnismal a squirrel, Ratatoskr, runs up and down the trunk carrying insults between Nidhogg at the roots and an eagle at the crown.
In the Voluspa Nidhogg also chews the corpses of the dead in a grim hall on the shore of Nastrond ("corpse-strand"), where oath-breakers and murderers wade through venom; and in the poem's closing stanza the dragon appears once more, flying up with corpses in its wings, in a scene whose place in the world's renewal after Ragnarok is debated. Nidhogg is less a character with a story than a persistent image of gnawing dissolution at the foundations of things.