Until February 27, 2023 at 6:59am PST / 9:59am EST (14:59 UTC), you can fight against a challenging 7-star Tera Raid featuring none other than Pikachu. Yes, Pikachu, the unevolved Electric-type mascot—although it is a Water Tera Type for this particular Tera Raid Battle. It comes with the Mightiest Mark, which grants it the title Pikachu the Unrivaled when sent out in battle.
While it might seem like a Pikachu wouldn’t be much of a threat, this Pikachu packs a powerful punch. It has the Lightning Rod Ability, which nullifies its weakness to Electric, meaning only Grass-type moves are super effective against it. It also begins the battle by setting up both Rain Dance and its shield, the latter of which mean affecting it with status moves is not a valid option. Additionally, the shield it sets up covers the entirety of its HP, meaning your only option for dealing meaningful damage against it is to Terastallize your Pokémon.
As for moves, it has Surf, Play Rough, Thunder, and Iron Tail. It also holds the Light Ball, meaning its Attack and Sp. Atk are much higher. Defeating it will grant you the option to catch it, once per game, and guarantee you TM123 Surf along with an Ability Patch on your first victory against it—you can fight it again, but you won’t be able to catch it if you already have, and you won’t get the guaranteed TM and Ability Patch (but you can still get an extra Ability Patch if you’re lucky!).
You can find out more information about this Pikachu over on Serebii.net’s page covering it.