Watchtower Lair's Den Stonjourner .

This Pokémon is encountered at the Watchtower Lair's Den when the event item is used.

These are the stats for this Pokémon:

Even though it was programmed for both games, this encounter is only valid for Pokémon Shield.
Important to note, the likely reason it's programmed in both games, so that the event raid is sharable to the other game.