Pokémon Sword and Shield brought about a new regional form for the ghost-type Pokémon, Yamask. Galarian Yamask evolves into a Runerigus.
To evolve your Galarian Yamask into a Runerigus, you have to make it take 49 or more damage and then take it to a specific spot in the Dusty Bowl zone of the Wild Area. Your Galarian Yamask’s level doesn’t matter, as long as it can live with at least 49 health points missing.
Have your weakened Yamask in your party and run under the stone arch in the Dusty Bowl shown below. After running under it with your Galarian Yamask, it will start to evolve.
If you’re having a hard time getting your Yamask down to enough health, take it to routes with the same level Pokémon and have it battle. Once you get close to 49 health points missing, you can take it to Route 2 or any area with weaker Pokémon to have a wild Pokémon whittle it down, ensuring that your Yamask will not die.
Keep in mind that damage dealt to your Yamask from weather, like sandstorms or hail, will not contribute to the total 49 or more damage. It seems like the damage does need to come from an enemy Pokémon.
The regular Unovan Yamask can be obtained from trading a Galarian Yamask to a kid in an Eevee outfit at the Ballonlea gym. Unovan Yamask evolves at level 34. The Yamask the kid trades you is level 36, so just level it up once to get a Confagrigus.