The official Pokémon website has announced that from July 23rd, 2021 until July 25th, 2021, at 4:59 p.m. PDT, Fossil Pokémon have invaded Pokémon Sword and Pokémon Shield’s Pokémon Dens and the Dynamax forms of the prehistoric Pokémon Omanyte, Kabuto, and Aerodactyl will be appearing more frequently.

Discover Dynamax Forms of Omanyte, Kabuto, and Aerodactyl in Max Raid Battles

Fossil Pokémon have invaded Pokémon Sword and Pokémon Shield’s Pokémon Dens, and they have a bone to pick with you.

Now here's a Max Raid Battle event you're sure to dig! Until July 25, 2021, at 4:59 p.m. PDT, Dynamax forms of the prehistoric Pokémon Omanyte, Kabuto, and Aerodactyl will be appearing more frequently in Pokémon Sword and Pokémon Shield's Pokémon Dens. You might event encounter a rare Shiny Omanyte while taking part in these Max Raid Battles.

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/discover-dynamax-forms-of-omanyte-kabuto-and-aerodactyl-in-max-raid-battles/