Spiritomb is an elusive Ghost/Dark-type Pokémon that was previously not available to capture or transfer into Pokémon Sword & Shield. It has finally been added in The Crown Tundra DLC and can be encountered and caught by trainers. However, finding it is not straightforward.
Where to find Spiritomb
Go to the Ballimere Lake area, near the Dyna Tree, in which a grave exists in the overworld. Interact with it, then make sure that Y-Comm has its online features enabled, and you must interact with 32 players that give you items within the Crown Tundra.
After completing that, return to the grave and Spiritomb will be where the grave was and you can catch it.