Yamask has quite a unique evolution method, which involves you meeting two different special conditions.
The first concerning the health of your Yamask and the second being visiting a specific location in the Wild Area.
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Where to find Yamask in Pokémon Sword and Shield
Yamask can be only be found on Route 6, which lies between Hammerlocke and Stow-on-Side.
You'll find Yamask happily walking around in the long grass waiting for you to attack it.
How to evolve Yamask into Runerigus in Pokémon Sword and Shield
If you want to evolve Yamask into Runerigus in Pokémon Sword and Shield, then you need to ensure that it takes more than 49 damage or more in one hit.
The easiest way to do this is to find a Pokémon in the Wild Area that is a good couple of levels higher than your Yamask, which should easily deal more than the required 49 damage.
We managed this with a strong looking Golurk in the Stony Wilderness.
You can also put Yamask in a battle with a Pokémon whose type Yamask is weak against. Though this does run the risk causing Yamask to faint.
The Pokémon types Yamask is weak against are:
Once the 49 or more damage has been dealt use a Pokémon Doll to quickly escape the battle.
Now it's time to head to the Dusty Bowl.
All you have to do here is head to the large archway that is at the far end of the Dusty Bowl and walk beneath it. The moment you step beneath the archway, Yamask will begin to evolve into Runerigus.
If you're having trouble finding the archway, probably due to a sandstorm, enter the Wild Area from Hammerlock.
From here head down the left-hand side of the hill, until sand begins to appear on the ground. Go into this sandy area and keep going forward, you should quickly arrive at the stone archway.
Sword and Shield finally here! Our now complete Pokémon Sword and Shield walkthrough and Pokémon Sword and Shield tips can help with the main adventure, and along the way you'll encounter a few new features - from the Wild Area and Pokémon Camp to strong-looking Pokémon and autosaving. After discovering the starters Sobble, Scorbunny and Grookey, we have a guide to the Turffield treasure riddle solution, and Ballonlea quiz answers and solutions, lists of all Pokémon Sword and Shield TM locations new Gen 8 Pokémon, Sword and Shield Pokédex, version differences, Galarian forms and Sword and Shield Legendaries. We also have an evolution guide, including guides on evolving Applin, Farfetch’d, Milcery, Sinistea, Toxel and Yamask.
How useful is Runerigus in Pokémon Sword and Shield?
Runerigus is definitely worth its complicated evolution method, because, not only does it look cool, it's a pretty useful Pokémon.
It can learn a variety of powerful ghost, ground and psychic-moves, which can make it incredibly useful for the later stages of the game. This includes the ground-type move Earthquake and Shadow Ball - a ghost type move.
If you're playing Pokémon Sword and are having trouble with the fourth, fighting-type gym, then Runerigus could be your saviour, since its immune to fighting-type moves.
Runeerigus can also use the move Mean Look, which will prevent wild Pokémon from running away. This is great for when your adding the more elusive Pokémon to your Pokédex.