Spiritomb is a secret encounter in The Crown Tundra and can be done at any point while playing.
Begin by fast traveling to Dyna Tree Hill, where you found the Galarian Legendary Birds. From there go East to Ballimere Lake (right on the map) to find a corner area with a tombstone. Interact with the tombstone and it will say:
“Spread my voice.”
Now, connect to the internet and talk to at least 32 fellow trainers. There is no in-game counter, so you’ll need to keep track of how many you’ve spoken to. While it may seem like an arbitrary number, this is a reference to the number of players you had to speak to in the Hallowed Tower to catch Spiritomb in Pokemon Diamond and Pearl.
The easiest place to find players nearby is next to the actual gravestone as others are trying to complete the secret task, or by the large tree at Dyna Tree Hill.
Once you’ve spoken to 32 trainers return to the tombstone and interact with it once more. If you’ve talked to enough trainers it will now read:
“My voice has been heard.”
Once you see this message, leave the area and return. You’ll need to actually fly out of Ballimere Lake.
Return to the tombstone and you should see Spiritomb waiting next to it.
Interact with Spiritomb to begin a battle.
Spiritomb is not shiny locked in The Crown Tundra.