This is a wild area event held by Sword and Shield. All the event slots are held by Gigantamax Pikachu.
This also marks the first time Gigantamax Pikachu is available in Max Raids as an encounter.
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.