Special items in Pokémon Sword and Shield make your life significantly easier. Many of them are useful because they allow certain Pokémon to achieve certain evolutions or improve their stats during combat. The Reaper Cloth is one of the items you need to evolve Pokémon, and the scarf of choice increases Pokémon's combat stats during a fight.
To start, you need to get an old letter from a little girl. You can find the individual in question in Hammerlocke. When you first arrive at the town entrance, go right and proceed along the path until you reach a set of stairs on the left. Go up, then go left. You should see a little girl standing next to the large building. He talks to her and says that she has a love letter to deliver to someone in Ballonlea.
Make your way to Ballonlea, and when you arrive in town, go to the left of the town's Pokémon Center and go up a small flight of stairs. You want the first house right after the stairs. Go inside and look for the older gentleman with blonde hair. Talk to him and he will tell you about how he was childhood friends with the individual who delivered the letter to you. This mission is like receiving the chosen scarf.
The chosen scarf is a useful item because it increases the Pokémon's speed stats. However, it restricts them to only using the first move they make in a battle. So if you have a Pokémon in possession of this item use tackle for the first attack during a battle, that's all it can do until the end of it. You have to think carefully about what move you want them to use.
You're not done yet, though. After receiving the chosen scarf, you must return to Hammerlocke and return to the place where you spoke to the little girl and started the search. When you come back, you see that she is no longer there. There is also no spark on the ground near her. You need to get close to where it was and spam the A button. Eventually, you should pick up a piece of Reaper Scarf after scouring this location long enough.
You must give the Reaper Cloth to Dusclops before trading it with a friend to make it evolve into a Dusknoir.