It is said that a mirror has been passed down from ancient times. Anyone who obtains this mirror can enter the mirror realm. In the mirror realm, you can get everything you want in your mind. But once you get what you want, your soul cannot leave the country. Hence the name, Dream Mirror.
Later, because there were too many people entering the country, they were taken away by a senior official to prevent anyone from entering the country again. Somehow, now, the mirror has spread again.