Nidhog
Nidhog is a Draki variety of Dragon. He is said to live at the base of Ygdrassil, and gnaw at the roots. His purpose for doing this is largely unknown, but he is a dragon that is typically said to want the destruction of the realms. When a plague, or illness sweeps across the land it is also said to be the poisonous breathe of Nidhog, even if the dragon is not seen.
The Norns, also said to reside at a well at the base of Yggdrassil are meant to keep the Draki at bay. How they manage this is unknown, but stories vary from lulling it to sleep, to feeding it.