The God War and Long Winter are treated as a single causal unit: the eruption that ended the God War triggered the volcanic winter that defines the Long Winter. Together they span roughly 75,000–45,000 years ago.
The God War: ~75,000 years ago
As people shifted from the material realm to Celestia, believers and prayers faded, and daemons began disappearing. The God War changed everything. Its cataclysmic expenditure of power shattered countless anchor-vessels across the material realm, stranding those in Celestia and triggering vicious competition for followers.
The combination of daemon survival war and gross expenditure of power on their followers resulted in total war across Alaria. Most people died; many others returned from Celestia and hid in isolation; others fought on.
The war's first and most deliberate campaign was waged not over land but over a class of priests. The anchor-vessel orders were the specialists who seated each daemon's presence on the material plane, and every faction grasped at once that a rival god could be reached by killing the people who held it there. So the factions hunted them. Sanctuaries that had stood neutral for ages were stormed for the binders inside, who took thirty to forty years to train and could not be replaced in the middle of a war. Within a single decade the orders were broken across the world, and Krathokh, the daemon of the binding rite itself, thinned and died before the eruption ever came. His loss is why the cold found the rest of the pantheon already failing: the anchor-vessel rite that had held every god to the world had stopped first.
The war ended when a follower of Dyos used all of his god's power to force an eruption of the Kajiit volcano, immediately killing millions more and beginning a volcanic winter that would set the world into a multi-millennia ice age.
The Long Winter: 75,000–45,000 years ago
As everything died and the ice age set in, small pockets of surviving civilization went into hiding in the mountains, surviving on lichens and hunting wild animals. No daemons survived, as the people of the material realm lost all faith in their gods. Hundreds of dragons died; many who survived terrorized the skies, hunting anything that could be found.
Technology regressed to a primitive state. The entire Golden Age pantheon of daemons was extinguished. The world would remain locked in ice until the rise of Craggus.