Embers of the long night
Everyone on the ashes believed that buried deep in some ruin in danger and famine there was a path to the new world, and that if you could find a unique key and open that door, you could enter the new world.
There, the earth is plenty, like milk and honey, and the sun is shining, as if it washes away the cold and gloom.
People no longer have to deal with desolation, monsters, infections, aberrations, and all sorts of dangerous things. There, children are happy, adults are happy, and everything is beautiful.
Every antiquarian, relic hunter, and history researcher on the ashes knows that this is the New World.