Game Freak's Pokémon Sword and Shield introduced new mechanics for hunting Pokémon that make use of the overworld spawning mechanic on routes and in the Wild Area. Pokémon actively wandering maps gave developers the opportunity to highlight certain Pokémon as special, and these special Pokémon occasionally spawn in Sword and Shield with a special golden aura, making them more noticeable when searching through grass or walking nearby. These unique spawns are called "Brilliant Pokémon," and they have a lot to offer a player who manages to capture one.
When first starting out in Pokémon Sword and Shield, Brilliant Pokémon spawn at random. However, as more Pokémon are captured, the chance for a Brilliant to spawn increases. By the time a player has captured 100 Pokémon, the chance for a Brilliant Pokémon spawn maxes out, offering a doubled chance to find Brilliant Pokémon in the wild. This milestone can be reached fairly quickly by capturing as many Pokémon encountered as possible.
Brilliant Pokémon have many special boosts that make them worth chasing. All Brilliant Pokémon have a guaranteed 2 maximum IVs, according to Serebii.net. Brilliant Pokémon possess a special egg move, which before Sword and Shield were only accessible through carefully breeding different Pokémon species together. This is good news for Pokémon fans who don't enjoy the arduous task of breeding and hatching numerous Pokémon of the same species.
Pokémon Sword & Shield's Brilliant Pokémon Have Special Boosts
Brilliant Pokémon also have a small increased chance of being Shiny. Because Shiny encounters increase when a player continuously battles or captures the same species, the two boosts often run side by side, offering Shiny Hunters a better opportunity of finding a Shiny Pokémon with good stats and moves. There are many Shiny Hunting methods in Pokémon Sword and Shield, but the basic chain-and-capture method appeals to Shiny Hunters who appreciate a grind when searching out these rare encounters, making these capture boosts worth the time for those who enjoy the challenge.
Encountering Brilliant Pokémon isn't limited to Pokémon who spawn in the overworld. They can also be found while fishing in any of the fishing spots scattered throughout Pokémon Sword and Shield. Much like with Pokémon that are wandering through the grass and walkways, Brilliant fishing spots will light up with a halo of golden light over the special location. Players only need to toss their Rod into the spot to encounter the Brilliant Water-type below the surface. Brilliant Pokémon encourage capturing and encountering Pokémon in Pokémon Sword and Shield, rewarding fans who continue to expand their Pokédex on their journey.
Source: Serebii.net