One Light Measures Mountains and Rivers

The story takes place in the "Twelve Crossings of Kyushu". Three hundred years ago, a black mechanism city called "Tianheng" fell from the sky. It is not spiritual, but it can deduce wars, disasters and personal choices from the words and deeds of a large number of people. All parties once relied on it to end troubled times, but later discovered that each deduction would take away a piece of the user's memory and cast the memory into a "wick" for the machine to continue learning. Tianheng was dismantled into nine lamps and divided into nine cities. Nowadays, "people of the same age" appear one after another in Nine Cities: people who don't know each other say the same words and make the same choices, as if they are replaced by some invisible answer.