The official Pokémon website has announced that from Thursday, October 29th, 2020, at 5:00 p.m. PDT until Sunday, November 1st, 2020, at 3:59 p.m. PST, players of Pokémon Sword and Shield can be on the lookout for spooky Pokémon like Cofagrigus, Trevenant, Gourgeist, Mimikyu, and Polteageist in Pokémon Dens.

Take on Trevenant, Mimikyu, Shiny Gourgeist, and More in Max Raid Battles

Be on the lookout for spooky Ghost-type Pokémon haunting Pokémon Dens in Pokémon Sword and Pokémon Shield.

The Halloween spirit is looming large over Pokémon Sword and Pokémon Shield. From Thursday, October 29, 2020, at 5:00 p.m. PDT until Sunday, November 1, 2020, at 3:59 p.m. PST, Ghost-type Pokémon will appear more frequently in Max Raid Battles. During the event period, be on the lookout for spooky Pokémon like Cofagrigus, Trevenant, Gourgeist, Mimikyu, and Polteageist in Pokémon Dens—if you're lucky, you might even encounter a Shiny Gourgeist!

To encounter these Pokémon in a Max Raid Battle, you'll need to connect your Nintendo Switch system to the internet through Y-Comm or by selecting Mystery Gift in the X menu, then selecting Get the Wild Area News. After that, whenever you see a pillar of light shining from a Pokémon Den, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle with a Dynamax Pokémon.

Source: https://www.pokemon.com/us/pokemon-news/take-on-trevenant-mimikyu-shiny-gourgeist-and-more-in-max-raid-battles/