How to Purify Shadow Pokemon in Pokemon Go
Shadow Pokémon have come to Pokémon Go. Catching one is relatively straightforward, much like a raid battle. However, with raids, there’s nothing left to do but raise your new teammate. Shadow Pokémon introduces an extra step in this process: Purification.
— Serebii.net (@SerebiiNet) July 22, 2019
Shadow Pokémon are poor, tainted things, thanks to whatever Team Rocket Go did to them. They have lower CP and know the move Frustration as a result of Team Rocket Go's actions. Purifying them will unlock its heart and power it up. According to Serebii, after capturing a Shadow Pokémon, players will have a screen similar to the normal Pokémon screen in their party, with the addition of a pink Purify button.
Purification costs Stardust and Candy but will return the Pokémon to normal. Serebii states that a Purified Pokémon will “gain multiple levels” and have a reduced cost on Stardust and Candy, plus they will forget the move Frustration and learn the move Return. These purified Pokémon will keep the move Return even through evolution.
All players have to do to evolve a Shadow Pokémon is to get enough Candy and Stardust together, and most veteran trainers likely already can afford it by now, especially since the list of available Shadow Pokémon is so short.