Through May 14, 2023 at 4:59pm PDT / 7:59pm EDT (23:59 UTC), you’ll be able to take on a 7-star Chesnaught in Tera Raids in Pokémon Scarlet & Violet. This Chesnaught is a Rock Tera Type and has the Mightiest Mark, which gives it the title “Chesnaught the Unrivaled” when sent out in battle.
Before you can challenge this Chesnaught, you’ll first need to have unlocked 6-star Raids, which is something you can do after beating the game and doing some of the postgame stuff. The Chesnaught raids will then appear in the Black Tera Raid Crystals on the map similar to the other 6-star Raid Crystals, though you’ll be able to tell it’s Chesnaught due to it being Rock Tera Type. Even if you don’t have it unlocked yet, you can still join other players if they have unlocked it and you have the Link Code or are playing with friends.
According to Serebii.net’s coverage, it knows the moves Earthquake, Hammer Arm, Stone Edge, and Wood Hammer, making it a very strong Physical-based attacker. It also uses Iron Defense, Bulk Up, and Curse as additional moves that it will use at various points throughout the battle, meaning you’ll have to watch out for it getting additional offense and physical defensive abilities and it’s a good idea to keep that in check. Additionally, it has the Bulletproof Ability, which protects it from popular moves you might consider using against it like Acid Spray, Aura Sphere, and Energy Ball, so keep that in mind.
If you’re looking for a guide for where to EV train your Pokémon to challenge Chesnaught, you can refer to Marriland’s Pokémon Scarlet & Violet EV Training Guides, which should help make your job easier by showing you which Pokémon to fight against to quickly and effectively EV train, although it’s up to you to decide which EVs and which Pokémon to use against Chesnaught (HP and Sp. Atk on Special-based attackers is probably the direction you’ll want to go, though, although you could also consider Defense).
After beating it, you can catch it and you’ll also be guaranteed to receive an Ability Patch and TM150 (Stone Edge) the first time you defeat it. You can only catch it once; after that, you can still fight it again for additional rewards, most of which are pretty good, but you won’t be able to get another Chesnaught.
This is the first of two runs of this event. Chesnaught will appear a second time the following weekend, from May 18, 2023 at 5:00pm PDT / 8:00pm EDT (May 19, 2023 at 00:00 UTC) through May 21, 2023 at 4:59pm PDT / 7:59pm EDT (23:59 UTC), although you will still be able to catch it just once per game across both events.