Chang'an in the Tang Dynasty was prosperous on the surface, but in fact the work of "monster census" was being carried out with great difficulty. Since Emperor Taizong signed the "Chang'an Contract" with all monsters, monsters can legally live in the human world, but they must be registered, abide by the law, and pay taxes on time. A secret yamen, the "Shanhai Bureau", which specializes in handling "Yokou" affairs, came into being.
