The Destiny Knot is one of the useful items for breeding in Pokémon Sword and Shield.
In the Sword and Shield versions, the latter is available for 10 Battle Points (PCo) in Hammerlocke or via Pickup ability but for those who want to get one even more easily, it's in the first extension that this happens.
Get the Destiny Knot on the Isle of Armor
To get this item, nothing could be simpler, you will just have to find the right island. Here is the map with its location to make your task easier.
Original content by Donovan"ZeroAk" Plault.