Datamined last month, Team Rocket and Shadow Pokémon have started appearing in Pokémon GO. At the time of writing, it appears they only appear at some PokéStops and for level 40 players for the time being – much like previous features such as Raids.

Thus far, we know the following:

• You must battle a Team Rocket grunt to have a chance at catching their Shadow Pokémon. They all have a purple aura and red eyes.
• Upon being captured, the Shadow Pokémon will know the move Frustration.
• It costs Stardust and Candy to purify a Shadow Pokémon.
• Upon purifying the Pokémon, Frustration will change to Return. Powering up and evolution costs from then on are decreased, as a reward for purifying.

We’ll update with more information as it filters through.

Thanks to Pichu for the heads up.

Source: Daily.pokecommunity