Military action
Heavily weakened by their blunder with the Second Abyssal War, the elves who occupied Eodur were in little position to defend themselves from a unified attack by the human kingdoms that aided in the demons' defeat, which is exactly what said kingdoms were banking on.
The elves put up a good fight, but when it became clear they simply didn't have the numbers to hold off the multi-fronted siege presented by the human armies, they surrendered the city and fled to the wilds of the Brytnian Woods.