Which river system's venom froze to form Ymir?

The venom that froze to help form Ymir came from Élivágar, the icy river system of Norse creation myth.

In the cosmology described in Norse sources, Élivágar flowed from Niflheim into Ginnungagap, the primordial void. Their poisonous drops, called eitr or venom, combined with the region’s intense cold. The resulting ice and venom produced the first giant, Ymir, also known as Aurgelmir.

The wording compresses several related details. In Vafþrúðnismál, venom drips from Élivágar and grows into a jötunn; Snorri Sturluson’s Prose Edda gives a fuller account involving ice, venom, and the void. Élivágar itself is not a single river but a collective name for dangerous primeval rivers.

Ymir later becomes the ancestor of the jötnar. After Odin, Vili, and Vé kill him, they use his body to shape the world: flesh becomes earth, blood becomes seas, bones become mountains, and his skull becomes the sky.