Purifying a Shadow Pokémon will improve its rating and reduce the amount of Stardust and Candy needed to upgrade, evolve, or learn a new attack. When purified, a shadowy Pokémon will forget about frustration and learn Return, a charged attack unique to purified Pokémon. To purify a shadowy Pokémon, once you've caught one by defeating a Team Rocket GO Grunt, you'll need to find it in your Pokémon list. Shadow search works, as does purification once you're done.
After capturing a Shadow Pokémon, players have the ability to purify a Shadow Pokémon. Purified Pokémon gain significant improvement in CP and IVs and lose damage mechanics increased by 20%. His charged attack also changes from Frustration to Return. They also have the added advantage of being easier to breed and evolve.