Icelost Years

The Icelost Years refer to an invasion of the continent of Tal'Dorei in approximately 537 PD by Errevon the Rimelord, an elemental behemoth of the Frostfell. During a celestial solstice, as the borders between planes weakened, the white dragon Skysunder helped Errevon to pass through the rift and conquer the continent of Tal'Dorei.

The Council of Tal'Dorei convinced Syngorn and Kraghammer to join in an alliance against Errevon, and together their forces battled Errevon back through the rift, which the Ashari druids sealed shut. The victory was later celebrated annually as the Winter's Crest festival.

Background
Following the long and devastating Scattered War, Tal'Dorei was significantly weakened. The white dragon Skysunder, acting as Errevon's scout to find the weakest points between the planes, used an existing tear between planes located in the forest that would later be named the Frostweald. She saw the battered and divided realm, and notified Errevon that it would be advantageous to strike. In approximately 537 PD during a celestial solstice (or the next winter solstice, or both), the forces of the Frostfell did so, further opening the rift and invading the Material Plane with the Algid Legion, Errevon's army of living blizzards and frost giants.

Known events
Initially, the denizens of Tal'Dorei in the path of Errevon's storms quickly fell, unprepared as they were for this conquest. The unending ice storm created by the Frostfell armies grew over several years to encompass a large section of the continent, and those trapped within it were forced to acquiesce to the Rimelord's demands or perish. At Foramere Basin, a confluence of several rivers, Errevon constructed a titanic a fortress of ice from which he ruled his new realm.

During these years Fort Daxio was built with the dual purpose of guarding against raids by the giant clans of the Cliffkeep Mountains and as a hidden reserve of military strength against Errevon.

In approximately 840 PD, after three years of Errevon's rule, 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.

Conclusion
The victory that ended the Icelost Years is celebrated on the winter solstice in Tal'Dorei, as Winter's Crest.

The geological and social changes wrought by this magical ice age remain. Errevon's ice fortress melted into the Foramere Basin, creating an important source of fresh water for central Tal'Dorei. Some dwarves and elves who fought in the alliance against the Frostfell armies settled near this lake not long after the war, and even many years later their descendants still live there. Because of the memory of that alliance, Foramere is known as a place where dwarves and elves get along well, in contrast to much of the rest of Tal'Dorei, and to this day elves and half-elves frustrated with Syngorn's prejudice often settle in the villages by the basin.

Skysunder was defeated and sealed beneath the Frostweald by the Arcana Pansophical, imprisoned within a giant crystaline egg. Because of this, the Frostweald remains in permanent magical winter, even despite Skysunder's release and defeat at the hands of Vox Machina centuries later.