This Pokémon is a rare variant that is capable of Gigantamaxing, and can only be obtained via Max Raid Battles in the Wild Area.
This rare variant can only be found in Pokémon Shield, though it is possible to invite others (including Sword) to join this Max Raid Battle.
While always available for capture (if found) in a den in the Wild Area, an event from January 09 to February 06, 2020 made it more commonly found in Pokémon Shield, by having it appear in dens normally not associated with it.
To be able to encounter the event variant, user's Switch must be connected to the internet, with the Mystery Gift>Get Wild Area News option used during the event period..
Alternatively user must have connected to the internet and redeemed a Mystery Gift during the event period.