Errevon

 was an elemental behemoth who attacked Tal'Dorei during the Icelost Years

The Icelost Years
Two years after the end of the Scattered War, Errevon and his elemental army invaded Tal'Dorei from the Elemental Plane of Ice via a portal in the forest now called the Frostweald. Errevon's elementals blanketed Tal'Dorei in eternal winter, and the behemoth constructed a citadel of ice in the Foramere Basin.

After several years, the Tal'Dorei Council was finally able to convince both Syngorn and Kraghammer - who had been on opposing sides in the Scattered War - to ally with them and fight back the invasion. Together, the peoples of Tal'Dorei along with Ashari druids of Zephrah were able to push Errevon and his servants back into the Frostfell. The Ashari permanently closed the rift between planes, and the storm then quickly subsided.

Published works

 * Critical Role: Vox Machina Origins
 * Vox Machina Origins Series III
 * Critical Role: Vox Machina Origins III #1