In the Northern Edda stories the serpent Nidhogg (Níðhöggr) is held within the roots of the world tree, they are one of the many serpents that exist within the underworld. The wyrms process the dead and Nidhogg, the ‘biter from below’ uses its poisonous bite to also gnaw at the roots of Yggdrasil, causing harm to the tree.
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