Ysildar

Ysildar was a gargantuan ice-blue serpent who was defeated by the vrykul in ancient times, according to the biased vrykul legend "The Wanderer and the Serpent".

Background
According to "The Wanderer and the Serpent", Ysildar slithered up from the "black pits of the earth" and hunted the vrykul during the time before the Breaking when Keeper Odyn often walked among the titan-forged in disguise. It had obsidian fangs and iron-like scales. It was so huge that when it stretched to its full length, its tail disappeared over the horizon, and it was capable of devouring a dozen vrykul at once. The creature is speculated to have either been one of Keeper Freya's animal followers, gone mad with rage, or a thing far more ancient and unspeakable, a nightmare born in the era of Shadow.

The vrykul eventually defeated it, taking out its eyes and blunting its fangs, but were unable to vanquish it for good. Odyn himself seized the gigantic serpent's tail, and with a mighty swing, hurled the beast skyward. The serpent flew so high that it passed across the sun and made day into night. Ysildar sailed through the sky for days until it finally landed in the sea and sank into the depths. The serpent was never seen again.

Speculation
Ysildar might be the reason serpents and serpentine creatures are associated with the Old Gods and their corruptive influence, like the Emerald Nightmare and the Druids of the Fang.