the walls kept tumbling down in the city that we love

New York City was no more. The Earth-was no more. The Fates did not foresaw it, so none ever anticipated it. Mortals brought the end of the world, it was slow going but by the time Rhea Jackson entered third year college, the world died.

With an offer of escape from the Fates, survivors of Camp Half-Blood took it, even if it means they're back to the far past. They'll take that over a dead world with no Gods.