The latest update to Pokémon Go adds Shadow Pokémon to the game. These mons are not so different than any other monsters, but they have succumbed to the anger and darkness to try to be stronger for their trainers, Team Rocket Grunts
These powerful Pokémon come know Frustration, a move that traditionally does more damage the more the Pokémon dislikes you, but once they purify them from their shadows, that move will turn into Return, a move that does more damage the more Pokémon loves you. However, Pokémon Go does not have Pokémon friendliness modifiers, so they both do a flat amount
How do I get a Shadow Pokémon
To catch a Shadow Pokémon, you have to go to a Knockout that's taken over Team Rocket. You will battle the grunt, and if you win, you'll get a chance to catch the Pokémon they were using.
How do I purify it
To purify the Pokémon, you just have to hit the button on their information screen.
Purified Pokémon and Pokemon I catch
Purified Pokémon cost less candy to evolve and less dust to power up. They also know Return, a normal-type attack. While the Return is pretty OK on a normal-type Pokémon, it's not super great on anything else and can be replaced.
Purified Pokémon also gets IV boosts when powered up. Every Pokemon can be a Shadow Pokémon
As of July 22, here's a list of Pokémon that can be captured as Shadow Pokémon ,