If you want to complete your Pokédex in Sword and Shield, you will need to do more than wander around catching Pokémon. Some of the rare Pokémon can only be through specific means, and some are hidden behind trades with NPCs. One such Pokémon is Unova Yamask.
The first thing you will need to get Unova Yamask is a Galarian Yamask. You can find one on Route 6, so head there, and you should be able to find one pretty quickly. Head to Baloonle Stadium, and you will find a trainer in an Eevee costume. Speak to them, and they will offer you Unova Yamask in exchange for your Galarian Yamask.
When you get your Unova Yamask, add it to your party and level it to level 38 to evolve it into Cofagrigus. You don’t have to do anything special or give it a particular item; it will just automatically evolve. This is the only Unova Yamask in the game; you cannot get more through breeding. Any eggs from an Unova Yamask will result in a Galarian Yamask. When you add this difficult to find Pokémon to your Pokédex, you will be one step closer to completing it.
If you need more help with Pokémon Sword and Shield, we have a range of guides that you might find useful. From how to use the PC Box Link, how to change a Pokemon’s nature, and how to trade Pokémon. If you are looking for some of the rarer Pokémon, we can show you where to find Dreepy and Ditto, as well.