The subject of this article has no official name. The name currently in use is a fan designator; see below for more information.
Legendary artifacts are items which are bound to specific Legendary or Mythical Pokémon. These artifacts typically do one or more of the following: allow the player to encounter these Pokémon, change their form, or improve their battling capabilities.
In Pokémon GO, the Mystery Box is used to obtain the Mythical Pokémon Meltan. When opened, wild Meltan will frequently appear around the player on a 30-minute timer. After the timer runs out, the box will close, and cannot be opened for another seven days.