There are many interesting Galarian forms of previous Pokémon in Sword and Shield. Their methods of evolving are certainly unique, especially when it comes to Galarian Yamask. If you thought evolving a level 30 Inkay from X and Y by turning your 3DS upside down was bizarre, here is how to evolve your Galar Yamask to Runerigus.
You will need to travel to Route 6 in order to obtain Galarian Yamask so it can evolve into Runerigus. This will be possible for you once you have earned up to three badges and are on your way to getting your fourth badge.
For the level, we recommend Yamask be at least 32. Once you got it, you are a few steps closer. The next method is tricky, which is to make sure your Galarian Yamask has taken more than 49 damage from its HP. For example, if your Yamask has 64 HP, you will need at least 15 HP or less in order to make this method work.
You can let your Yamask do some battling against wild Pokémon in order to make it to that threshold, but using a revive on it after fainting or using a move that will inflict damage to itself like Curse will not work. Location does not really matter, but you can be in the Wild Area to save yourself time. After receiving more than 49 damage, you can run away from the wild Pokémon fight.
After that, head over to Dusty Bowl, which is below Hammerlocke. Then you will go to your left and travel over to Dusty Bowl until you will see a stone sculpture which is a rock structure that looks similar to Stonehenge from England. When you enter underneath the giant rock sculpture, it will trigger the Galarian Yamask to evolve into Runerigus, regardless of which level it is.