A new Pokémon variant in Sword is an ice type of Black and White Darumaka and Darmanitan. The evolution process is slightly different than Black and White, however, as you will need an Ice Stone to be able to evolve Darumaka into Darmanitan.
How to evolve Darumaka into Galarian Darmanitan in Pokémon Sword
To find the Ice Stone, you can try your luck at this beautiful spot on Outrage Lake, or you can reach Route 9. You will need a Roton bike with the Surf function to get one of them, and Route 9 will give you a Stone of Ice guaranteed, while the Lake of outrage can be the luck of the draw.
To find the Frozen Stone on Route 9, simply go down into the water, then go to the upper right corner and follow that path around until you see something sparkling in the water. It will be a stone of ice.
Once you have the Ice Stone, you can use it on Darumaka and turn it into Darmanitan. You don't have to worry about leveling them, there is no level requirement for evolution to occur.
If you need further help with Pokémon Sword and Shield, we have a number of guides that you may find useful. From how to use PC Box Link, how to change the nature of a Pokemon, and how to trade Pokémon. If you're looking for some of the rarer Pokémon, we can show you where to find Dreepy and Ditto too.