Pokemon sword Y Sign introduced a new regional form for the ghost Pokémon Yamask. Galarian Yamask evolves into Runerigus.
In order to evolve your Galarian Yamask into Runerigus, you must make it take 49 or more damage and then take it to a specific location in the Dusty Bowl area of the Wild Area. Your Galarian Yamask’s level doesn’t matter as long as it can live with at least 49 missing health.
Weaken your Yamask in your party and run under the stone arch in the Dusty Bowl shown below. After walking under it with your Galarian Yamask, it will start to develop.
If you’re having trouble getting your Yamask to reach enough health points, take it to the streets with Pokemon of the same level and battle it. Once you get close to 49 missing health, you can take it to Route 2 or any area with weaker Pokemon for a wild Pokemon to knock it down and make sure your Yamask doesn’t die.
Note that damage done to your Yamask by inclement weather such as sandstorms or hail does not contribute to total damage of 49 or more. It looks like the damage must have come from an enemy Pokemon.
The normal Unovan Yamask can be obtained by exchanging a Galarian Yamask with a kid in an Eevee outfit at the Ballonlea Gym. Unovan Yamask evolves at level 34. The Yamask the child trades for you is at level 36, so you only need to level it up once to get a Confagrigus.
Source : polygon.com