The Eviolite is a held item that boosts its user's Defense and Special Defense by 50%. However, this effect only works if its holder can still evolve. It's a good item to have when battling tough trainers in Pokemon Scarlet and Pokemon Violet.
There are two ways to get Eviolites in Scarlet & Violet. Once players have reached the post-game, they'll be able to buy as many Eviolites as they want as long as they have the money. For those that are still progressing through Scarlet & Violet's main story, they can get their own Eviolite free of cost by battling.
South Province (Area 5) Trainers
If players head to the far southeast of Poco Path Lighthouse, they'll reach a shoreline with a Pokemon Center. This is South Province (Area 5), and it's where Scarlet/Violet players can get the Eviolite. Speak to the Elite Four Rep and he'll want players to defeat 7 trainers in South Province (Area 5). This means trainers fought at the beginning in South Province (Area 1) will not count towards the Eviolite reward.
Before Scarlet/Violet players head to South Province (Area 5), they may want to unlock Koraidon/Miraidon's ability to "jump higher" from the Path of Legends storyline. This will make it easier to reach some of the trainers without having to circle around. Also, players will want to make sure their team is around level 21-23, as the trainers in South Province (Area 5) have Pokemon from level 19 to level 23.
(refer to the above image to find 7 trainers)
Once this is completed, return to the League Rep at South Province (Area 5)'s Pokemon Center to get the Eviolite. This is the only way to get the Eviolite before the post-game. After Scarlet/Violet's end credits, players will be able to buy an Eviolite from Delibird Presents for 50,000 poke dollars.
Who Can Use Eviolite?
The Eviolite itself can be held by any Pokemon, but it's effects only work on Pokemon that can still evolve. This means it can't work on Pokemon like Flamigo, who can't evolve, and it won't work on Legendary Pokemon such as Koraidon or Miraidon. It can work on Pokemon like Pikachu or Eevee, who still have yet to evolve into Raichu or one of the Eeveelutions respectively.
Eviolites are best equipped to a Pokemon with solid Defense and Special Defense. Although Scarlet/Violet players are no doubt in a rush to evolve their Pokemon, it may be best to hold off for some species, as they benefit from the Eviolite more than they would as their final evolution. One example is Chansey, a Normal-type Pokemon that evolves into Blissey. While Chansey (and Blissey's) Defense is extremely poor, it has an incredibly high HP and amazing Special Defense. A Chansey holding an Eviolite will be sturdier than an itemless-Blissey, since Eviolite increases the Defense and Special Defense of a Pokemon's total stat rather than its base stat.
Thus, Eviolites should be given to un-evolved Pokemon that have good Defense/Special Defense or need a Defense/Special Defense boost. They may be good to equip to a Pokemon prior to taking on difficult Tera Raids. Keep in mind that like most items, the Eviolite is not immune to the Dark-type move, Knock Off.
Pokemon Scarlet and Pokemon Violet are available for Nintendo Switch.