Nidhogg

Nidhogg, the malice-striker, is the great serpent-dragon coiled in Nastrond beneath Yggdrasil. He gnaws ceaselessly at the deepest root of the World Tree, working to topple the cosmos from below. Among the corpses he chews are oathbreakers, murderers, and seducers cast into the venom-river of the dead. The squirrel Ratatosk runs insults between him and the eagle in Yggdrasil's crown. In Voluspa, after the world is reborn, Nidhogg still flies up from the dark fells bearing corpses on his wings, an old terror that does not die.