This is a Wild Area event held by Sword and Shield. Besides Zeraora, all the event slots are held by Pokémon that can Gigantamax.
This event allows Zeraora to spawn as event boss, though important to note that it cannot be caught.
This event also features the shiny Zeraora, at a higher level of difficulty.
If a million Zeraora gets defeated before a deadline, a shiny Zeraora will be gifted to players via HOME.
To qualify, you need to shift a mon into HOME during that period.
This is the first wild area event since the DLC Isle of Armor dropped.
It adds a new file, called normal_encount_rigel1.
It either means players with the DLC can have their own event raids (across all of Galar),
or these event raids only apply to Isle of Armor.
To import these files into your save, use PKHeX's block importing function. Our tutorial should guide you through it.
Our first post of this page would contain some data relevant to the importing stage.
There's info and some common questions regarding Wild Area Events here.
To see other Wild Area Events, here.