Short Version of the Legend
The Crown is an ancient, powerful magical artifact that is said to have been made from a star by a God (or Goddess?) who's name has long been forgotten. It was made into the shape of a Crown for the First Yōkai Emperor, but it will magically adapt it's form to the Emperor who owns it for the duration of their reign.
It was the source of both peace and war amongst Yōkai. At the era where Yōkai no Sekai happens, peace has been going on for several centuries - but who knows how long this peace will last?
The Crown gives unique, temporary powers to the Emperor which can be found here.