In a Tera Raid Battle, you’ll join a team of four to try to defeat a powerful Tera Pokémon before time runs out. Tera Raid Battle events where you will be able to take on specific Pokémon or encounter Pokémon with specific Tera Types are in development and will make their debut sometime after the games are released.
https://scarletviolet.pokemon.com/en-us/news/black_crystal_tera_raid/