The official Pokémon website has announced that players of Pokémon Sword and Shield can encounter Gengar, Drifblim, Chandelure, Gigantamax Gengar or Shiny Chandelure in Max Raid Battles from August 20th, 2021 until August 22, 2021, at 4:59 p.m. PDT.

Encounter Gengar, Drifblim, Chandelure, and More in Max Raid Battles

Ghost-type Pokémon are haunting Pokémon Sword and Pokémon Shield, and you might even find a Gigantamax Gengar or Shiny Chandelure.

Ghost-type Pokémon are sneaking out of the shadows to haunt the Galar region's summer nights. Until August 22, 2021, at 4:59 p.m. PDT, Gengar, Drifblim, and Chandelure will appear more frequently in Pokémon Sword and Pokémon Shield's Max Raid Battles. Very rarely during this event, you might even encounter a Gigantamax Gengar or Shiny Chandelure!

For an opportunity 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. Usually, there'll be an extremely small possibility a Gigantamax Pokémon will appear instead of a Dynamax Pokémon, but during an event, certain Gigantamax Pokémon are more likely to appear. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/encounter-gengar-drifblim-chandelure-and-more-in-max-raid-battles/