From June 25th, 2021 until June 27, 2021, at 4:59 p.m. PDT, players of Pokémon Sword and Pokémon Shield can encounter Octillery, Malamar, and Grapploct in Max Raid Battles and have a chance to find a Shiny Grapploct.

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Encounter Octillery, Malamar, and Grapploct in Max Raid Battles

Tentacled Pokémon lurk in Pokémon Dens during this Pokémon Sword and Pokémon Shield event, and you might find Shiny Grapploct.

Be careful when poking around Pokémon Sword and Pokémon Shield's Pokémon Dens, as you're liable to find yourself in the grip of a powerful tentacled Pokémon! Until June 27, 2021, at 4:59 p.m. PDT, Octillery, Malamar, and Grapploct will appear in Max Raid Battles. And very rarely, you might even encounter a Shiny Grapploct.

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. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/encounter-octillery-malamar-and-grapploct-in-max-raid-battles/